‘Maine had spoken to Miss Cost outside the church. She was telling Mrs Carstairs she would go to the Spring after the service and collect the necklace that had been left on the shelf. She ran after Maine and Mrs Carstairs went into church. We don’t know what passed between them but I think she may, poor creature, have made some final advance and been rebuffed. She must have armed herself with her horrid little snapshot and list of dates and been carrying them about in her bag, planning to call on him, precipitate a final scene and then confront him with her evidence. In any case she forced them on him and very likely told him she was going to give the whole story to the Press.’

‘Did she – ?’

‘Yes. It was in the mailbag.’

‘You said, I think, that you did not normally intercept Her Majesty’s mail.’

‘I believe I did,’ said Alleyn blandly. ‘Nor do we. Normally.’

‘Go on.’

‘He knew she was going to the Spring. He was no doubt on the look-out as he washed his hands at the sink in the Trethaways’ cottage. He saw Wally. He probably saw you pin up your notice. He saw Barrimore tear it down and go away. He went up and let himself in. He had admittance discs and used one when we sent for him. He hid behind the boulder and waited for Miss Cost. He knew of course that there were loose rocks up there. He was extremely familiar with the terrain.’

‘Ah, yes.’

‘When it was over he scraped away his footprints. Later on, when we were there, he was very quick to get up to the higher level and walk over it. Any prints that might be left would thus appear to be innocuous. Then he went back in his launch at ten minutes past eight and waited to be sent for to examine the body.’

‘It gives me an unpleasant frisson when I remember that he also examined mine,’ said Miss Emily. ‘A cool, resourceful man. I rather liked him.’

‘So did I,’ Alleyn said. ‘I liked him. He intended us, of course, to follow up the idea of mistaken identity but he was too clever to push it overmuch. If we hadn’t discovered that you visited the Spring, he would have said he’d seen you. As it was he let us find out for ourselves. He hoped Wally would be thought to have done it and would have given evidence of his irresponsibility and seen him bestowed in a suitable institution, which, as he very truly observed, might be the best thing for him, after all.’

‘I shall do something about that boy,’ said Miss Emily. ‘There must be special schools. I shall attend to it.’ She looked curiously at Alleyn. ‘What would you have done if the lights had not failed, or if you had caught up with him?’