I can try and direct you if you like, we went looking for it last year and found what Chance found. I'll be honest, I didn't post the pictures because I really wasn't sure that we had found it as it reminded me more of a ruined house than a barrow...
I've just checked on the map and with the memory bank (the gf) and we parked somewhere about ST 795 628 by a gateish and then shinyed over a gate. Walked along a track from the gate heading north running parallel to the road which goes up to A road which then joins the A4. Stay within 20 yards or so of the road whilst you walk if poss. Then we stumbled across the remains.
As a side note, its a very posh area and our battered car with england flags (it was during last years world cup and I have no shame) got some very dirty looks and absolutely no courtesy from the chelsea tractors we met en route. I'll have a quick dig about and see if I can find the picture with the gf in it which shows the height of the walls.
