Parked about 20 yards from main entrance last season and then had to fight my way out, inch by inch, through the massed ranks of Bristolians. Got lots of dirty looks, no-one would get out the way, so I engaged in some loud revving, got a tiny window of opportunity and went for it.
My advice...anywhere else will do.
coming in from the motorway past filton airfield follow the signs and then turn left off the main road and pass the ground (on your right) then take the next right (at the traffic lights) - loads of free parking along there and also in the side roads