If you were looking for an international drugs empire, Downham Market would not be the first place you’d think of. With a population of around 10,000, this sleepy Fenland town is probably about as typical as they come — typical, that is, apart from the smell.

It was around two years ago that residents first noticed it: a distinctive pungent scent which seemed to hang on the wind before eventually engulfing the town for several days.