Firstly: check the weather forecasts before you head along there. If there’s been any significant amount of rain in the last few days or over the event, expect mud. Or, rather, Mud.
As suggested getting off the ground is excellent. An airbed, a campbed, or even just a large layer of something squishy UNDER you is worth more than OVER you. Consider sticking a blanket, your spare kit, your cloak, your towel under your sleeping bag.
Extremities. Toes are one that gets me, pack a large warm pair of bedsocks. And keep them dry. You may also want to look at a nightcap (warm wooley hat) of some sort.
Beyond that, thermal underwear, a good sleeping bag, a reasonably insulated tent, and possibly a (small) hot drink before bed.
It might even be possible to get a hot water bottle filled before you turn in: there will likely be a fair few tea shops if not the caterers.
This sounds really extreme. And it’ll probably not get TOO cold. But many here remember the very first Empire, where the Mud froze beneath us… And it’s better to overpack and have spare warm stuff, than to have to stumble along to First Aid…