‘Down which side of the table?’ asked the judge.

‘That’s the point,’ said William. ‘The same side.’

‘As I thought,’ said the judge. ‘But I’m still only half-way there.’

‘When the flask was returned to Hamish, he made a great show of taking another swig from it. His second mistake.’

‘I’m lost,’ said Beth.

‘Patience,’ said William. ‘Concentrate on the seating plan and all will be revealed. James Buchanan, my recently promoted Detective Sergeant, spent this morning identifying all the fingerprints he could find on the tumblers, wine glasses and coffee cups of everyone who had sat on the left-hand side of the table, while I carried out the same exercise with Hamish’s hip flask.’

‘You’re still a yard ahead of me,’ said Catherine. ‘If Hamish’s flask was passed to his father at the top of the table, everyone’s fingerprints on that side of the table had to be on it.’

‘But they weren’t,’ suggested the judge, ‘because someone had switched the flask for a similar one before it reached the chairman at the top of the table, and that person will be the only one whose fingerprints were on both flasks.’

‘Not a bad summing up, m’Lud,’ said William with a grin. ‘So in order to decide who is the guilty party, the jury must first consider the evidence. Flora took Hamish’s flask and passed it on to the doctor like a baton in a relay race, and when the chairman sent it back down the table, the same exercise was carried out in reverse. Simple and well planned, except the accomplices made two mistakes. First—’ William stopped mid-sentence as Franco reappeared to clear their plates.

‘Would you like to see the dessert menu?’ he asked.

‘No, thank you,’ said Catherine, not even looking up.

‘A digestif, perhaps?’ ventured Franco.

‘No, thank you,’ repeated the judge a little more firmly, his eyes still fixed on the place settings on James’s drawing. Franco left, having served no purpose.

William waited to see if anyone had worked out what those mistakes were.

‘Whose fingerprints did you find on Hamish’s hip flask?’ asked the judge. ‘And, more important, whose fingerprints were missing? Because that will tell us who switched the flasks.’

William acknowledged the judge with a slight bow, as if they were in court. ‘The only fingerprints I could identify on Hamish’s flask were Flora’s, who was seated next to him, the not so good doctor Lockhart, and of course Hamish’s.’

‘Got it,’ said Beth.

‘Then you’ll be able to explain what they were up to,’ said William.

‘It had to be Dr Lockhart who carried out the switch both times,’ said Beth. ‘Otherwise, the chairman’s fingerprints would also have been on Hamish’s flask when it was passed back to him.’

‘As well as those of James’s mother, who was sitting between the doctor and the chairman, and passed the flask to him.’

‘Have you found the second flask?’ asked the judge. ‘The one Fraser Buchanan must have drunk from?’

‘Yes,’ said William. ‘I first saw the other flask in Dr Lockhart’s bag when I questioned him last night, and then again this morning when I searched his cabin while he was at breakfast. But the only fingerprints I found on that particular flask were his.’