I was casting around for an idea.

I’d had several, all for historical novels built around real events, but they petered out after a page or two. Browsing in The Curious World of Samuel Pepys and John Evelyn by Margaret Willes – lots of excellent illustrations – I came across this passage about the 1665 Great Plague:

The final death toll according to the bills of mortality was 68,576, but in reality around 100,000 perished, about one-fifth of the city’s population. … The disparity in numbers was probably caused by the fact that the ‘searchers of the dead’, usually old women employed by parishes to examine the corpses and ascertain the cause of death, had little or no medical knowledge and therefore could only give unreliable diagnoses.

I had found my novel.