We spent an afternoon at Wrest Park near Bedford. It is owned by English Heritage and consists of a mansion with several well restored rooms to visit as well as recently restored gardens and park land and a lake. The gardens are extremely attractive and the parkland has attractive buildings and statues amongst the trees and lawns.
Many of the paths are suitable for wheelchairs and there is also a "bus" to take the disabled or elderly to various points in the grounds.
I think it would also be a good place to visit with grand children as a special audio guide is available for children and it is a good place for them to run around looking for the next building or statue. There is also a children's playground and cafe/shop near the car park.