Kelly doesn't hold back on gory details - opening the book with a body found in a block of ice on the Fens and a corpse "riding" on a stone gargoyle high on Ely Cathedral. But he had to be tactful to avoid offending Ely's teachers.
"There is a school in The Water Clock and I wanted it to be really quite rough. I live in Ely and there is only one secondary school there. In order not to offend anyone, I had to create two schools, one inside the ring road, which is the real one, and one outside, which is rough," said Mr Kelly, who has a young daughter who may eventually go to the real school.
He is now finishing a sequel to The Water Clock, called The Fire Baby, again starring detective and journalist Philip Dryden ("a nod to Marlowe"). A key character will be a female teacher called Estelle. "I am struggling to make sure that the character isn't like Estelle Morris," Mr Kelly says. We look forward to reading the result.