There's a trackway you can park on, it runs by the car park. If it's full just hop on a grass verge. I've been to a few of these and never had too much trouble parking, sometimes you have to walk a bit if you're parked on the road.
Of course, things would be so much easier if they'd open the car park, but that would mean going to all that trouble of unlocking the bloody gate, wouldn't it?

Have fun, you'll love it! XX