Pipers Hill Farm fits the bill for hundreds of acres of off road hacking - as well as access to some great bridleways, and they've got a nice menage and do all year turnout. Don't think they do DIY, but certainly do part livery, and it's a lovely quiet yard - mostly horses in to break so not too...
If you haen't managed to find anywhere yet, I can highly recommend Pipers Hill Farm between Bishops Itchington and Gaydon (5-10 mins from Lighthorne).
I think they only have full or part liveries, but it's a lovely quiet yard, 300 acres of offroad hacking with some xc and hunt fences, nice...