Parked there very recently. No problem, park in the area prior to picket point, register car at guardroom, drive to mess and park. Out of the gate, turn left to Ruislip Gardens (5 minutes walk).
My daughter has a dependant's pass and has also parked there when going for an interview a couple of months ago. I rang the guardroom before she went but was told that I didn't need to as she had a pass.