Good to see a bit of activity on this request after all this time, especially as I still haven't tracked down the two William Durhams. It's amazing how common the name is!

Thanks for your assistance Bob. I have now found the family who were at Clarendon Square in 1861 and Eversholt St in 1871 living at 28 Carlton Street, St Pancras in 1881. Mary Ann Durham is the head and Edgar, John and Laura are living with her. I know Wm Durham junior was living in Hartlepool in 1881 so this could still be the family but I can't make the link. The younger Wm Durham's age varies considerably in later censuses and at his death in 1937 and I haven't managed to find his birth.
Good point about Harcourt St. I had spotted that myself and oddly enough Wm Durham's wife's birthplace is given in 1881 as "Brown, Marlebone", Brown St being right next to Shouldham St. It would be interesting to know which parish covers the area, as presumably that is the meaning of St Andrew, St James etc in the 1861 census.
Regards
Geoff