Mrs Hogarth coughed to get my attention again. 'This memo was drafted by the Defence Ministry. I don't know who did it, but junior ministers will have seen it in the early stages. Perhaps it was drafted many times. That could add up.'

'I'm interested in who saw the document or a copy of it,' I said.

'Well, let's look at what might have happened to those eight copies of the memo,' she said briskly. 'In each minister's private office there is his principal private secretary plus one or two bright young men. Additionally, there will be an executive officer and a couple of clerical officers.'

'Would all those people normally see a memo like this?'

'Certainly the PPS would read it. And one of the clerical staff, or perhaps an executive officer, will file it. It depends how keen and efficient the others are. I think you should assume that all of the people in each minister's private office would have a good idea of the content, just in case the minister started shouting for it and they had to find it.'

'Sounds like a lot of people,' I said. The gardeners were lining up the newly planted roses, using a piece of white string.

'We're not finished yet. The Cabinet Office, the Home Office and the Foreign Office would all have executive responsibilities arising from this document.'

'Not the Home Office,' I corrected her gently.

'That's not the way they'd see it,' she said. Obviously, she too had had dealings with the Home Office, who assumed executive responsibility over everyone and everything.

'You're right,' I said. 'Please go on.'

'So in those departments the memo would go to the permanent secretary, and to his private office, and then to the appropriate branch to be dealt with.'

'Two more administrative officials and at least one executive or clerical officer,' I said.

'In the Cabinet Office add one private secretary and one executive or clerical officer. From there to the Defence Secretariat, which would mean three administrators and one executive or clerical officer.'

'It's quite a crowd,' I said.

'It adds up.' She drank some tea.

A man came in through the door. 'I didn't know you were in here, Mabel. I was just going to use the phone.' Then he caught sight of me. 'Oh, hello, Samson,' he said.

'Hello, Pete,' I said. He was a baby-faced thirty-year-old, with light-brown wavy hair and a pale complexion upon which his cheeks seemed artificially reddened. For all his Whitehall attire – pinstripe trousers and black jacket – Pete Barrett was a very ambitious career policeman who'd taken a law degree at night school. He'd adapted to local costume in just the way I would have expected when I'd first met him about five years earlier. Barren was a Special Branch man who'd been desperate to get into the Department. He'd failed to do so and despite this soft job he'd found, he was bitter about it.

'Is that man bothering you, Mrs Hogarth?' he enquired with his ponderous humour. He was cautious about baiting me, but it was a diffidence laced with contempt. He went round to the window, looked out at the garden as if he might be checking on the gardeners, and then looked at the papers on the desk. She closed the spiral notebook in which she'd been doing her figuring. It had a double red stripe on the cover; such notebooks are for classified information with all the pages numbered.

She kept her hand on the closed notebook. 'A routine enquiry,' she answered, in a studied attempt to discourage his interest.

But he was not to be deterred. 'A routine enquiry?' He gave a forced chuckle. That sounds like Scotland Yard, Mabel. That sounds like what I'm supposed to say.' He leaned forward to read the document on the desk in front of her. He held his tie against his chest so that it wouldn't fall against her. This stiff posture, hand flat on chest, his wavy hair and red cheeks made him look more than ever like a puppet.

'If you're after tea, you're unlucky. My girl is off sick, I made it myself this afternoon. And my ginger biscuits are all finished.'

Barrett didn't respond to this at all. In other circumstances I would have told him to go away in no uncertain terms, but this was his territory and I had no authority to be asking questions here. And I could think of no convincing reason for having this copy of the memo. Furthermore I had the feeling that Barrett had known I was in the room before coming in.

'A Cabinet memo no less,' he said. He looked at me and said, 'What exactly is the problem, Bernie?'

'Just passing the time,' I said.

He stood upright, a puppet on parade now, chin tucked in and shoulders held well back. He looked at me. 'You're on my patch now,' he said with mock severity. Outside, the two gardeners had dug the line of holes for the roses, but one of them was looking up at the sky as if he'd felt a spot of rain.

'It's nothing you'd be interested in,' I said.

'My office received no notice that you were coming,' he said.

Mrs Hogarth was watching me. She was biting her Up, but I don't know whether this was in anger or anxiety.

'You know the drill, Bernie,' he persisted. 'A Cabinet memo… that's a serious line of enquiry.'

Mrs Hogarth stopped biting her lip and said, 'I wish you'd stop reading the papers on my desk, Mr Barrett.' She put the photocopy memo I'd given her into the tray with other papers. That particular paper was nothing to do with my visitor and I find your reading it aloud a most embarrassing breach of security.'

Barrett went red. 'Oh…' he said. 'Oh. Oh, I see.'

'Use the phone next door. There's no one in there. I really must get on now. Perhaps you're not busy, but I am.'

'Yes, of course,' said Barrett. 'I'll see you around, Bernie.'

I didn't answer.

'And please shut the door,' Mrs Hogarth called after him.

'Sorry,' he said as he came back to close it.

'Now where were we,' she said. 'Ah, yes: Number Ten. Here in Number Ten such a memo would be handled by two private secretaries. And one executive or clerical officer must have seen it. And I think you should consider the possibility that the press office and policy unit were interested enough to read it. That would be quite normal.'

'I'm losing track.'

'I have a note of it. I haven't added the Defence Ministry people…' She paused for a moment to write something on her pad, murmuring as she wrote, '… private office, let's say two; permanent secretary's office, another two… and policy branch, plus clerical. Let's say eleven at the Defence Ministry.'

'Eleven at the Defence Ministry? But they had no executive action.'

'Don't you think they would want to notify their units in an effort to keep these SIS intruders out?'

'Yes, I suppose they might. But they shouldn't have done it. That wasn't the idea at all. The plan was intended to test the security.'

'Don't be silly. This is Whitehall. This is politics. This is power. The Defence Ministry is not going to stand there and wait patiently and do nothing while you cut their balls off.' She saw the surprise in my face. She smiled. She was a surprising lady. 'And if you're going to do a thorough investigation, you must take into account that some ministers have private secretaries who would handle all papers that cross their ministers' desks. And the way that papers are filed in a registry sometimes means that the registry clerks handle them too.'

'It's a hell of a lot of people,' I said. 'So even the most secret secrets are not very secret.'

'I'm sure I don't have to mention that papers like this are left on desks and are sometimes seen by visitors to the various offices as well as by the staff. And I haven't included your own staff who handled this particular one.' She tapped the photocopy lightly with her fingertips.

'That particular one? What do you mean?'

'Well, this is a photocopy of the Cabinet secretary's copy. You knew that, didn't you?'