Have you read Adrian Plass's autobiography Janet? Of all his books I think that's my favourite, it's more serious and poignant than his fiction. I also read Silver Birches recently. If you liked his other stuff you might like that too, I thought it was pretty good on the whole (It's also been published under the title Ghosts). I agree that Leonard Thin is weaker than the Sacred Diary, which I remember being quite funny, although it's years ago since I read them.