“The police description of him says that he is in a brown suit.”

“Yes, because Cayley told the police. You see, even if Mark had had lunch in his brown suit, and the servants had noticed it, Cayley could always have pretended that he had changed into blue after lunch, because only Cayley saw him afterwards. So if Cayley had told the Inspector that he was wearing blue, Mark could have escaped quite comfortably in his brown, without needing to change at all.”

“But that’s just what he did do,” cried Bill triumphantly. “What fools we are!”

Antony looked at him in surprise, and then shook his head.