Welcome Ni to mumsnet and camping! Lots of friendly people here so please ask away. I haven't been to Just So, but I'm sure I saw an old thread on here about it the other day/week so try searching or alternatively someone who has been may chime in.
Re tents, the bell tents are lovely. Consensus seems to be go for a 5 meter if you afford / have space for one. And people seem to have had good experiences with SoulPad. We are thinking of getting one but not sure if we can justify it. We have a Vango Samara 600 which is a great tent, especially for UK weather as it has a massive living area for days when you and DC may be stuck inside. nice big windows for watching the rain, and a spare bedroom for storing all your bags out of the way. The Vango Icarus is very similar, but on a smaller scale and is very popular with the mumsnet crowd.
For essentials, for me, sleeping things is mandatory so SIMs for the 4 of us (though we can get away with 3 if need to) and sleeping bags and pillows if we have room in the car. If you don't have sleeping bags and have room in the car, you could just use a duvet, especially if the night isn't looking particularly cold.
For food, particularly at a festival, you have a lot more options. You could go minimal and have nothing amd just expect to buy all your meals. I would probably bring my trangia which is good for cooking for the for of us, but not so great if we are camping with friends. And some plates, etc. And my icey-tek to store all the food and beer/wine (ESSENTIAL)
If we are talking non essentials then you can bring tables and chairs and picnic blankets and fairy lights. There are a few threads on packing lists if you do a search, but I would try to bring the bare minumum which would be something to sleep on, something to eat from, and a light.
Hope that helps x