Since I had an afternoon in sunny Dereham, here’s what The Concise Oxford Dictionary Of English Placenames has to say about the history of the town name.
Dereham, Norfolk. Derham in Domesday Book, Estderham in 1428. Likely the Old English, Deorhamm, for an enclosure for deer.
There are two Derehams in Norfolk, the larger of the two is also known as East Dereham and West Dereham is around 25 miles away, nearer to Downham Market. A nice and simple town meaning, which has remained gloriously unchanged (well, nearly) for a millennium.