I don't see any reason to think Sarah Dore was anything other than an "ordinary" girl from Darenth. In the C18th it wasn't unusual for rich men to live quite openly with their mistresses. Arthur Hill obviously accepted paternity as William was given the surname Dorehill. I don't know who the other illegitimate children were who he acknowledged in his will, or who there mothers were, but I don't read anything more into the acknowledgement of the illegitimate children and their financial support than that he was "doing the right thing by them"
The real Arthur Hill sounds a very decent kind of man (according to the morals of the day) and his reputation is taking quite a battering by the TV series.
Carole