'There is some supportive evidence for that theory,' said Rees, glancing at his notes. 'There was a discharge of semen in the victim's underwear, consistent with high sexual excitement. It may even be the case that some of the bruising was a deliberate part of any ritual. There are – believe it or not – people who actually derive pleasure from pain and who pay others to administer it.'

Stockdale grinned. 'You don't need to tell me that, Tegwyn,' he said. 'When we raided a house in Charlotte Street last month, we found a man hanging naked from the rafters while a woman in a black mask flayed him with a cat o'nine tails.' He pulled a face. 'I don't mind telling you that it's not my idea of pleasure.'

'We're still working on assumptions,' Colbeck reminded them. 'It would be mistake to build too much on them. Explain one thing to us, Dr Rees, if you can,' he went on. 'Even someone who enjoyed pain to a certain point would surely have cried out when he was struck on the head with a blunt instrument that broke open the scalp.

'You're quite right, Inspector.'

'Then why did nobody hear the noise?'

'You should have been there when I examined the back of the victim's neck,' said Rees, loftily. 'There were unmistakable marks of something having been tied very tightly against it. My considered opinion is that, before he was killed, the victim was bound and gagged. He could neither move nor speak. The gag was only removed when the acid was about to be poured down his throat.'

'That would explain the cry for help,' said Colbeck. 'One of the guests heard it as she walked past and it was quickly stifled.' He gritted his teeth. 'Whoever committed the murder did not simply wish to kill Hugh Kellow. They were determined to make him suffer.'