Oh wow, I've just seen that Berkshire Archives say that they have found out that Harriot's mother was called Seraphina Donclere who died in 1810 and that they have a copy of her will:
https://www.facebook.com/berkshirere...91269454217038
Apparently there is also a copy of the will on FMP - I'll post a link in a minute if I can find it.