Take your normal skirmish kit, obv. And a sense of humour, it probably wont be the best quality of play you have ever experienced but it is a laugh in its own right and the quantity of people is really something else.
I would play fairly light if I was going again, the heaviest things would be a decent container of water, I took 3l in a camelback and finished it every day ( it was very hot the two years I went) and gun. Go for a few high caps, I took 3 and with so many people there it is easy to step back and refill them. Ammo, a bottle of gas if you must but I didn't bother, you are unlikely to need a secondary as most of the fighting is at range so a couple of spare mags will probably do. you are not allowed burning pyro that's anything standard really, BFG's are pretty much pointless as well so I wouldn't bother carrying them.
The site is massive so my other recommendation would be to have your mobiles fully charged for your team to contact each other to meet up when you inevitably become lost or separated. Also the PMR 446 (non licenced frequencies) become jammed with incessant call tones and people being dicks, the working range drops due to the hills, valleys and trees.
Reducing your need to return to the camp site during play is always a good thing so I carried a small tool kit, with things like spare fuses, a few essential allen keys, a Leatherman, a spare battery and a short length of net curtain rod stuff, you know the metal spring wrapped in plastic stuff, it coils up small and works great for unjamming guns.
Carry a tenner so that when you come back to the camping area you can visit the bar and pick up a cold drink, mmmm beer!
We bbq'd Friday and Saturday, there is a Sainsburys close by so 1 car doing a beer and meat run on sat afternoon means there is less to go rotten while you are playing.
If there are enough of you going, pooling for a private porta loo is quite good we had 15 of us so I think it worked out at something like £5 or £10 each, and a combination lock means you can look after it, one guy brought some toilet wipes and a couple of air freshener blocks. It was all rather civilised.