Umber knew that if he worked at it he could reduce the figure to zero. Philip Dormer Stanhope, 4th Earl of Chesterfield, was a septuagenarian relic of a bygone political age when Junius began writing his letters. He was plainly too old and too ill to have been responsible for them. Philip Francis was an obscure War Office clerk, too young, some would say, and too low in the pecking order to be the author. Lauchlin Macleane was an unprincipled Irish-Scots adventurer with a taste for political intrigue. The Scots were, however, routinely abused by Junius and Macleane himself attacked in a letter generally reckoned to have been written by Junius under a different pseudonym. Richard Grenville, 2nd Earl Temple, shared many of Junius's prejudices, but was the brother of George Grenville, with whom Junius engaged in private correspondence without any apparent fear of recognition.

Modern historians had settled on Philip Francis. His opinions, his character and his whereabouts fitted Junius like a glove. A computer-aided stylo-statistical analysis had also fingered him as a habitual user of Junian phrases and constructions. His youth and his junior station counted for little in the face of that. Case closed.

Not quite. The handwriting was the problem. There was no similarity at all between his and Junius's. This was explained away by most experts as evidence of Francis using a disguised hand when writing as Junius. Fine. But Junius wrote fluently and elegantly, while Francis scratched away crabbedly all his life. The disguised hand should logically have been inferior to the real thing, not the other way round.

Amanuenses entered the argument at this stage. And certainty went out by the opposite door. Francis seemed too secretive a man to have employed an amanuensis and nobody could suggest who he might have chosen for the role in any case. Meanwhile, some graphologists detected a similarity between Junius's handwriting and that of Christabella Dayrolles, wife of Lord Chesterfield's godson Solomon Dayrolles. Thus, bizarrely, the finger of suspicion took a late swing back towards its least credible target – a half-blind, stone-deaf and largely bedridden old nobleman, who had died two months after Woodfall's receipt of Junius's very last letter.

Christabella Dayrolles. The name chimed distantly in Umber's memory. Yes, that was it. She was the subject he had been working on at the end of the Trinity term at Oxford in 1981. She was the seemingly trivial point his researches had arrived at, never, in the event, to progress beyond. He could recall little of what he had learned about her and there was nothing in any of the books he had consulted to assist him. If he still had his boxful of Junius papers, it would be a different matter. But he did not. Christabella Dayrolles was, for the moment, out of reach.

'What do you know about her?' Sharp demanded when Umber explained the problem to him during their drive to Mayfair late that afternoon.

'Precious little. Her husband was a career diplomat and a favourite of his godfather. Chesterfield's letters to Dayrolles are a treasure trove of information on Georgian politics and court life. Mrs Dayrolles was… Dayrolles's wife. Mother of his children. Keeper of the domestic flame. Stereotypical eighteenth-century female. Or not. I don't know.'

'But her handwriting resembles Junius's?'

'Yes. Superficially, I seem to remember. More than Philip Francis's does, that's for sure. But Chesterfield as Junius? I could never buy that.'

'What about her husband, then?'

'Dayrolles? He's never been in the frame.'

'Why not?'

'Because…' Umber hesitated. It was a good question. And there was a good answer, he felt certain, though he could not for the moment recall what it was. He had been trying to connect Mrs Dayrolles with Junian suspects other than Lord Chesterfield when he had abandoned his researches in the summer of 1981. His efforts had taken him nowhere – as far as he knew at the time. But perhaps they had taken him closer to the truth than he could ever have suspected. 'I'm going to have to go back into it, George. That's all I can tell you.'

'Well, maybe you won't have to, if Oliver Hall gives us a lead.'

'Yeah,' said Umber half-heartedly. 'Maybe.'

Umber did not expect anything to have changed at 58 Kingsley House. But Marilyn's absence and Oliver's presence turned out to constitute more than a simple swap of hosts. The atmosphere was cooler, almost chill. There were fewer lights on. There was no music. The tone of everything was palpably different.

Umber remembered Oliver Hall as a quiet, reserved, smartly suited man in his early forties. He had less hair than a couple of decades before and what there was of it was grey. He had developed a slight stoop and a turkey neck. He was wearing what Umber took to be his idea of casual dress – razor-creased trousers, cashmere sweater, check shirt. He looked neither relaxed nor nervous. He did not offer them a drink. He made no overtures. They had his attention. That was all.

'I never expected to see either of you again,' he said when they had sat down. 'It's doubly surprising… to see you together.'

'I assume you've spoken to your former wife about our visit to her,' said Sharp.

'Oh yes. I'm fully apprised.'

'What about Sally's attempt to phone her just before she died?' put in Umber. 'Did Questred tell you about that?'

'You can assume I know everything I need to know,' Hall replied.

'She didn't by any chance try to phone you as well, did she?' asked Sharp.

'Not as far as I know.'

'Can't you be sure?'

'No.'

'Why not?'

'Because she might have phoned me, got no answer and failed to leave a message.'

It was a precise and incontrovertible answer. Hall's lawyer would have been proud of him. It gave nothing away – except, of course, his reluctance to give anything away.

'Has Radd's murder made you doubt his guilt, Mr Hall?' Umber asked.

'No.'

'It's made Questred doubt it.'

'I can't say that surprises me. Edmund's a rather woolly thinker.'

'What about your son?' asked Sharp. 'How does he feel about it?'

'The same as I do, I imagine.'

'You imagine?'

'We haven't actually discussed the matter.'

'Don't you think you should?'

'I'm sure we will. At some point. Jeremy clearly isn't concerned about it. Otherwise he'd have contacted me.'

'You don't see a lot of each other, then?'

'As much as we both want, Mr Sharp. Neither more nor less.'

'Relations with a step-parent can be difficult, I'm told. Maybe your remarriage… put some distance between you.'

Hall smiled faintly, as if amused by the blatancy of Sharp's attempt to prod a nerve. 'No. It didn't.'

'How does he get on with… Mrs Hall?'

'Very well, thank you.'

There was a magazine lying on the table in front of Umber. It was the Culture supplement of the Sunday Times, folded open at the theatre reviews page. The RSC production of All's Well That Ends Well at the Gielgud had been given a chunky write-up. He could not stop himself glancing down at it before he looked across at Oliver Hall, who was already looking straight at him. 'Has she come over with you?' Umber asked in as idle a manner as he could contrive.

Hall nodded. 'Marilyn's in London with me, yes.' Once again the studied accuracy of his statements was apparent. She had not come over with him. He had not claimed as much. But it was the inference he would happily have let Umber draw.