So, totally not in Happy Valley, at all...
I actually just stumbled across this, whilst looking for a location from 'Royal Warriors' and I'm totally chuffed to have done so. I'm not sure what is there now and this road has certainly changed, but it is still recognisable as the location of Lydia Shum & Bill Tung's pizza shop and apartment, although their apartment seems to have been turned into a shell - probably something to do with the road which has been built next to it.