The constable led her along the corridor and tapped on the commander’s door. Responding to a bellow from inside, he opened the door and announced: ‘Miss Harry for you, sir.’ Greatly daring, he followed added: ‘I hope you have a very pleasant evening, sir.’

‘Thank you, constable. I’m sure I shall.’

The exchange of male shibboleths was undetectable. The men were too professional to allow a knowing smile or a raised eyebrow to give them away.

She’d arrived exactly on time. Joe was busy with a cigar at an open window, discreetly puffing smoke out in the direction of Horse Guards. Gaze on the middle distance, tails, white tie, severely simple shirt and waistcoat, he caught himself posing and came forward to welcome his guest, then stood and stared at her in astonishment.

He realized he’d been silent for longer than was polite. ‘Great heavens, Wentworth! Look at you. Anyone would think you’d just stepped out of a Fabergé Easter egg!’

‘Drat! I knew I should have worn the gymslip!’ he could have sworn she mumbled.

‘No, you misunderstand! Oh, please don’t droop! Shoulders back, chin up, constable! I meant it as a compliment. You look like something designed by the world’s best jeweller. Sleek, precious, unique. A knockout! And that greenery-yallery colour is very … very …’

‘Fresh and fashionable, sir?’

‘Exactly! I couldn’t have specified anything better if it had occurred to me to do so.’ He fiddled about, extinguishing the cigar he’d just lit and frowning. ‘Not in the habit of advising on female attire, unless it happens to be uniform which I’d consider within my province. Forgive me! In fact, I think you look just perfect. But how on earth could you get it so right? Did you know that …’ He gave her a searching look. ‘There’s no way you could possibly …’

‘Know? I know nothing yet! Are you ever going to tell me what exactly you want me to do this evening, sir? I’m really not at my best being run in blinkers.’

‘Of course. Impossible to speak earlier for very good reason. Orders! But now I think I can come clean. That’s why I asked you to get here early. And the first thing – you must call me Joe for the duration of the duty … when we are in company, of course.’

‘I’ll try to remember that, Commander.’

The telephone on the desk rang and he made a dive for it, realizing he was glad of the diversion. ‘No. I went home half an hour ago. You should do the same, Ned. Bring this to me on Monday.’ It rang again the moment he replaced the receiver. ‘Yes, but I’m engaged. Well, that’s a nuisance but it will have to wait until next week.’

He turned back to the Lily. ‘Look – as long as I’m here in my office, people will try to get hold of me.’ He unhooked the receiver from its stand and put it on the desk. ‘That’ll do for a start. But we’ll find somewhere else for your briefing. Somewhere discreet … What about the cocktail lounge in Claridges? They have a useful little alcove or two there … potted plants … That suit you?’

Lily nodded.

‘Good. Good. But before we leave – one little thing. Sit down, will you?’

Joe opened the top drawer of his desk and took out a document. Two or three sheets were paper-clipped together. He passed them across to her along with his fountain pen, uncapped and ready. Always an uncomfortable moment. You could never tell how people would react to this ceremony. ‘I’d like you to sign at the bottom on the dotted line.’

She put the pen down and began to read.

He interrupted. ‘Move it along, Wentworth! It’s just a formality. What you have there is a copy of the Official Secrets Act. By signing, you’re simply promising to reveal no state secrets … cross your heart and hope to die and all that. Breathe a word of what transpires tonight and I’ll stick you in the Tower.’ He feared his dismissive grin was not reassuring. She ignored him and read on.

‘Commander, this is unnecessary,’ she announced at last. ‘I’m an Englishwoman. My father is a war veteran. My grandfather was wounded in South Africa, fighting in a cavalry regiment for his country. My word – which I’ll readily give – should be good enough for anyone. I see no reason to sign such a document. It’s pointless anyway. Don’t you think I’d be hurrying to sign with an innocent smile and a contemptuous flourish if I were an anarchist … or a Communist … or a Fenian?’

The three words were delivered slowly. Joe guessed she was testing his reaction to one of these current bugbears of law and order. He recognized a game he’d played himself.

‘Instead of which you’re digging your heels in, fussing about details and threatening to ruin what could be a perfectly good evening. Champagne, caviar and Cecil Cardew’s band complete with crooner are all on the menu. To say nothing of the company of the most eligible bachelor in London.’ He allowed time for that to sink in. ‘Are you sure you want to sacrifice that spectacular dress for a technicality?’

After a moment he reached over, took up the sheets and put them away in a drawer, sighing. ‘Very well. We’ll just have to take for granted your loyalty to the State. It makes not a scrap of difference. Step out of line, Miss Wentworth, and someone … someone with more clout and bigger boots than mine … will settle your account.’

She didn’t seem to mind. ‘Now that sounds entirely reasonable to me. I’ll agree to that,’ she said. ‘And, in return, you have my word that I’ll do and say nothing that could – as far as I understand it – endanger the state. But tell me more about this bachelor. Not, I’m assuming, yourself?’

‘You assume correctly – if discourteously. If a list of such things were kept, I believe I’d feature at about number five hundred. And sinking weekly. The gossipmongers have rarely found me of interest and now I’m pleased to say they appear to have given up on me. The last time the hounds of the press noticed me I was billed as “back from India still a bachelor Sandilands”. And we all know what that means. It’s a degree worse than last season’s “confirmed bachelor”. It sends a clear message to mothers of marriageable daughters. “This one’s survived the Colonial Fishing Fleet – he’s clearly a hopeless case!” Not that many would welcome a policeman into the branches of their family tree. It’s not only the criminals that the words “Scotland Yard” send rushing for cover.’

He was chattering – still uncomfortable with his briefing task. He battled on. ‘No. You’re to look on me as no more than your escort – your chaperon for the evening – which, if all goes according to plan, you will spend in the close company of the aforementioned bachelor. Now – let me check – can you dance? Foxtrot? Quickstep? That sort of thing? Not a detail one finds mentioned in the files.’

‘Five years of Saturday mornings at the Stretton Academy of Tap Dance and Terpsichore. It didn’t seem relevant information for my application form. I dance adequately but I’m no Adele Astaire.’

‘Should be good enough. Now – your partner for the evening is an exceptionally good dancer. I’ve seen him performing. He’ll steer you around the floor all right. And his name’s David. He’ll expect you to call him David when you’re alone together.’

Lily’s voice was chill with suspicion. ‘I think I begin to see why you checked my height and weight. Should I be thankful that I chose to wear low-heeled shoes, sir?’ She stuck out her right foot for his inspection.

‘Ah! You’ve guessed.’ He made a show of examining her foot. ‘Not too high, not too low. Good choice. Calfskin, would they be?’ This was bluff and bluster, but Joe couldn’t help indulging in it to cover his unease. She waited for him to get to the point. ‘Well, don’t try running off in them before midnight, will you?’ His tone was playfully apologetic. He even wagged a finger. ‘Your partner is full of youth and vigour and keeps late hours. You’re to stay locked in a tango with him for as long into the night as he wishes.’