A church art exhibition turns deadly . . .''Art, my dear boy,'' said Mr Askern, ''especially sacred art, needs tradition. Tradition is the bedrock of our art . . .''He broke off, staring at the woman in front of him. Her face seemed to lose all definition and her skin turned an unnatural shade of putty-coloured grey. ''Art,'' she said, her voice scarcely more than a whisper. ''Art! Oh my God, art!''She swayed dangerously. Jack leapt forward, catching her as she fell.Jack Haldean expected Lythewell and Askerns'' exhibition of church art in Lyon House, London, to be a sedate affair. After all, Lythewell and Askern, Church Artists, were a respectable, old-fashioned firm, the last people to be associated with mystery, violence and sudden death. Or so it seemed - until after the exhibition . . .