'This is a very serious matter, Mr Tenison,' Tartaglia replied firmly, holding his gaze.

Tenison sighed. 'Of course, and I said I was sorry.' As if that were the end of the matter.

Tartaglia and Donovan sat opposite Tenison in the confined space of interview room eight at Belgravia Police Station, tape machine running. Viktor Denisenko had led them to Tenison's flat a couple of hours before and they had then tracked Tenison down to a charity do in a hotel where he was one of the speakers. He was still in his dinner jacket and bow tie and looked a little tired and the worse for wear. There were no grounds yet to arrest him, but he was now being treated as a person of interest and he had insisted on having his lawyer, Geoffrey Mallinson, there too. Mallinson sat beside him, red-faced and puffed up like a fat bullfrog, ruffled and bleary-eyed from interrupted sleep.

Perhaps, as a politician, Tenison thought himself above the law, but if so he was mistaken. It was true that people lied all the time in investigations, often for the most silly and innocent of reasons. For some people, lying was an automatic reaction, not because they had anything specific to hide but because a lie was sometimes easier and less time-consuming than the truth: I wasn't there; I didn't see anything; I don't want to get involved. People also lied in murder cases because the stakes were higher. But if someone like Tenison lied about something as innocent as having dinner with his sister, it meant he was hiding something else, or at least so Tartaglia's instincts told him.

'At the very least, I can charge you with obstruction,' he said, still studying Tenison's broad, impassive face as though somehow it might give away a glimmer of the truth.

Tenison spread his hands. 'I've said I'm sorry. What more can I say?'

'What I don't understand is why you lied to us, Mr Tenison. Why didn't you just tell us right at the start that you had dinner with your sister on that Thursday night?'

Tenison frowned, as though it were obvious. 'It's none of your business, that's why.'

'Wrong. This is a murder investigation, Mr Tenison. Everything is our business.'

Tenison leaned back in his chair, took a deep breath and said, as if explaining an obvious fact to a very small child: 'What I meant was, Rachel was killed the following morning. I had nothing to do with that.'

'We only have your word for where you were. You have no alibi and you've lied to us once already. That calls into question everything that you've told us.' Tartaglia spoke slowly and deliberately, giving weight to each sentence.

Under the harsh glare of the strip lighting, Tenison looked visibly shocked, as if it hadn't occurred to him how his behaviour might be interpreted. There were dark shadows under his eyes and the few lines on his face seemed more deeply drawn. Perhaps events were taking their toll, or perhaps it was down to the formal clothes he was wearing, but he looked older and more worn than when Tartaglia had visited him at his flat.

'Right. Let's run through what happened that evening.'

Resignedly, Tenison ran a palm over his sleek, dark hair and gave Tartaglia a weary look. 'I had a quick drink with Rachel at her flat, then we went out to dinner.'

'Just for the record,' Donovan asked. 'What did you have to drink at the flat?'

Tenison glanced over at her, as if he'd only just noticed her there. 'I really can't remember, Sergeant. Why, is it important?'

'A number of dirty glasses were found in her flat from that night,' she replied. 'It would be useful if you could tell us which one was yours.'

'Well, I probably had a glass of wine. White, if I remember correctly. I am trying to help, you know.'

Tartaglia nodded. 'Please continue with your account, Mr Tenison.'

'We had a drink, then caught a cab to the restaurant, as we were running late. We were in the restaurant about an hour. Then we had an argument, as you probably already know, and Rachel left.'

'What was the argument about?' Donovan asked.

Tenison shrugged. 'Personal stuff, mainly.'

'I'm going to need the details, Mr Tenison,' Tartaglia said.

'Is this really necessary?'

'Yes. I need to understand everything that was affecting Miss Tenison in the run-up to her murder.'

Tenison looked away towards the small, barred window in the corner, drumming his fingers lightly on the table. 'If you must know, I'd been having some marital difficulties.' He spoke so quietly, Tartaglia almost missed what he said.

'Could you speak up, Mr Tenison. For the tape.'

Tenison turned around and glared at him. 'I said I was having some marital difficulties. Is that loud enough for you?'

'Yes. Thank you.'

Again Tenison looked away, avoiding eye contact. 'Rachel thought I was going to leave my wife and she was trying to stop me.'

'How did you react?' Donovan asked.

A flicker of embarrassment or possibly pain crossed his face. 'I told her it was none of her business. Rachel, being Rachel, wasn't satisfied with that. This wasn't the first time we'd spoken about it, but she wouldn't let it rest. When she had a bee in her bonnet about something, she just wouldn't give up. In the end, most people, including me, gave in to her.'

'Was your sister close to your wife?' Tartaglia asked, wondering what sort of grown man would allow his stepsister to dictate his life. It struck him again that Tenison was rather weak, certainly as far as his sister was concerned.

'Not especially. Although Rachel approved of Emma, they are I should say were very different types of women.'

It wasn't clear to Tartaglia if Tenison meant this as a criticism of his sister or of his wife, but he had the feeling it was the latter. 'So if Miss Tenison wasn't championing your wife's cause, why did she care so much about what you did?' Before he could answer, Tartaglia guessed the reason. 'There was another woman involved, wasn't there?'

Tenison winced as though he had tasted something bitter. 'Rachel didn't want me wrecking my life for what she saw as a fling.'

'Is that what it was?' Donovan asked.

'It's none of your bloody business what it was.'

'It is, where it concerns Miss Tenison,' Tartaglia said. 'You were telling us that she saw your relationship as something trivial?'

Tenison hesitated. 'Probably. She certainly thought I was being foolish.'

'Foolish?'

He spread his hands. 'Come on, Inspector. We've all done things on the spur of the moment without thinking them through properly, things we've lived to regret.'

Tenison didn't strike Tartaglia as the impulsive type and he wondered what had led him to take such a risk, both personally and professionally, and if he was trying to play down what had happened for their benefit. However, they were straying from what was important.

'Let's get back to the restaurant. What happened after Miss Tenison left?'

'I left some money to cover the bill and got a taxi back to her flat. But she wouldn't let me in, so I went away.'

'You didn't persist?' Donovan asked.

'What was the point? It was late. I was tired. We'd had arguments before and it was nothing new.'

'What time did you give up and go away?'

'I really can't say. I guess it would have been about ten-thirty, or so. I didn't exactly look at my watch.'

'Where did you go?' Tartaglia asked.

'Back to my flat, of course.'

Tenison's tone was off-hand and dismissive and it struck a false chord.

'Nobody can corroborate that.'

Tenison gave the faintest shrug, as though it was of no concern.

'I think you're lying.'

Tenison's jaw set hard. He folded his arms and leaned back heavily in his chair. 'I don't give a damn what you think. That's what I did. You prove otherwise.'

Tartaglia shook his head slowly. 'What makes much more sense is this: you followed her home as you say. You were angry with her. Maybe your relationship was closer than you say it was...'

Tenison's face flushed with anger. 'What the hell are you implying? I loved my sister, but not in that way, I can assure you.'

'Well, somebody slept in Rachel Tenison's bed that night,' Donovan said. 'Are you saying it wasn't you?'

'God, you lot are twisted. Contrary to the squalid little stories you read in the gutter press, not everyone's into incest.'

'You went round to her flat and she wouldn't let you in,' Tartaglia continued. 'Then you find out she's got another bloke in there so you wait until the next morning when she goes out for her run and you kill her in a fit of jealous rage. You wouldn't be the first man to do it.'

'Hang on a minute,' Mallinson said, springing to life. 'This is all quite ridiculous. You have no proof and may I remind you that my client is here voluntarily. If you persist with this sort of questioning, he will leave.'

'You killed her, didn't you?' Tartaglia insisted, ignoring Mallinson.

Mallinson put a hand on Tenison's sleeve. 'They have nothing, Patrick. They're just fishing.'

Tenison shook his head. 'I'm fine, Geoffrey. You're letting your imagination run away with you, Inspector.'

'Really? I wonder how the press will see it. The gutter press, as you call them. As you say, they're used to things good and lurid, particularly where politicians are concerned.'

'It's trial by press now, is it?'

Although Tenison was trying to appear calm, Tartaglia had seen the split second flicker of horror on his face when the word 'press' had been mentioned. It again confirmed what he had suspected: Tenison had something to hide. He had to keep running with it. It was his only lever.

'If you won't help us, Mr Tenison. Well...' He opened his hands.

'That's blackmail.'

'Not at all. You lie to us about a simple matter of a dinner. We discover that you're the last person to see Miss Tenison alive, bar someone she sleeps with and someone who murders her. The simplest explanation is that you did all three things.'

'Stop right there, Inspector,' Mallinson interjected. 'My client has told you why he didn't come forward sooner. It doesn't make him guilty of anything.'

Still Tartaglia held Tenison's eye. 'Who knows what goes on between two people who appear to be close, what the real dynamics are. Contrary to what you say, it's not a huge stretch of the imagination to see you as either her lover or her killer.'

Tenison looked away and shook his head as though the whole thing were ridiculous.

Tartaglia wasn't sure what he believed, but if Tenison had murdered his sister it didn't explain the presence of the poem at the crime scene, let alone the other links with the Watson case.

Mallinson cleared his throat. 'This is a wild flight of fancy, Inspector, and you know it. Unless you have anything more constructive to say, I advise my client to leave now.'

'Your client has lied to us, Mr Mallinson. Therefore everything he says is open to question. We know they had an argument. Maybe it was about something more than Mr Tenison's little affair. Maybe he's still angry with her the next day. So he follows her into the park and she won't speak to him. He's really frustrated by now. They have another fight and he strangles her. At the very least, we're looking at a charge of manslaughter.'

Tenison scraped his chair back and got to his feet, his face contracted with anger. 'You're wasting my time. You've got no evidence and I'm going home. All I can say is that I loved my sister and I didn't kill her.' He turned to walk out of the room.

'Then what is it you're trying to hide?' Tartaglia called after him. 'We'll find out sooner or later whatever it is, as will the press. It's amazing how these things leak out. Is that what you really want?'

Tenison stopped just before the door and looked round.

'My client has done nothing wrong,' Mallinson repeated.

'In that case, why is he still lying?'

'Come back here for a moment, Patrick.' Mallinson patted the chair beside him. 'I'm sure we can sort this out amicably.'

'All I want is the truth,' Tartaglia said quietly.

'I am telling the truth, for God's sake,' Tenison shouted, his face red again. 'I didn't kill her.'

'All right, Mr Tenison. Please sit down and let's try and clear this up.'

Tenison stared at him for a moment, then shrugged. He came back over to the table and sank down grudgingly in his seat again, arms tightly folded.

'OK. Let's go back to the matter of your alibi. Once more, where were you between seven and eight a.m. the following day?'

'For the umpteenth time, I was at my flat.'

'What time did you leave that morning?'

'About eight o'clock. I caught a train from Waterloo down to my constituency.'

'That's what, about a forty-five minute journey at most?'

Tenison nodded.

'But your PA says you arrived just before eleven and that you called ahead to cancel your first couple of meetings. Why were you late?'

Tenison gritted his teeth. 'Was I? I don't remember.'

'Look, it's very easy for us to check the CCTV footage to see what time you got to Waterloo. From what I can tell, you had ample time to kill Miss Tenison. If you didn't do it, you'd better give me a better reason for why were you late.'

'I overslept and couldn't get a taxi. OK?'

'Not good enough.'

Mallinson wrapped the table with his knuckles. 'Come, come, Inspector. Do you have any evidence to link my client with the crime scene?'

Tartaglia turned on Mallinson. 'I want the truth about where he was and either I get it out of him now, between these four walls, or I'll let the press do it for me. He can take his pick.'