I can't believe this site isn't already on TMA as it is so easily accessible, in a beautiful place and it's easy to see the remaining parts of it.
In Burnham Beeches, an ancient woodland full of huge, gnarly, old beech and oak pollards, the hill fort has been quite badly damaged due to quarrying and WWII vehicle activity. The banks and ditches are clear to see where they remain. Beware that this part of the wood is grazed - I saw some very pretty cows, some ponies and a tiny deer (muntjac or Chinese water deer)!
I didn't go there specifically to see this site - the forest alone is worth a visit (especially if you're into large, ancient trees) and the hillfort is an added bonus! The forest feels bigger than it is, I think because you can't see out of it.
Free parking in the week. I expect it gets very busy at weekends. Definitely worth the trip though!