Some Hyde Park / Kensington Gardens House Sparrow census figures;
Nov 1925: 2,603
Dec 1948: 885
Nov 1966: 642
Nov 1975: 544
Feb 1995: 46
'The reasons speculated for the national decline - changes in farming practices and increased number of predators, including Sparrowhawks - do not seem to apply to the parks unless the population, considered by some as largely sedentary, does require an injection of birds from outside London. The decline in horse traffic after 1925 has also been suggested as a possible factor'. (Sanderson. 1995).
My last sighting was of a single in 2006 though four birds were reported in January 2010 on one of the areas set aside for the RSPB's House Sparrow project. These areas now appear to be defunct from what I can see.
As for Sparrowhawk; a pair probably breed annually (but ascertaining this has been surprisingly difficult). Sanderson states that there wasn't a single record of this species between 1953 and 1975 (though this in part will be down to observer coverage) and only seven records between 1975 and 1995.