OP, you will know if it’s possible it might work as a holiday or not (will your DC actually enjoy it or will it be torture for them?). If it’s not going to work then sack it off and tell him to put the tent back on eBay (he is an utter eejit for managing to buy a non-refundable £700 tent on eBay! Absolutely no need for that especially as you didn’t have any input into what tent you bought. I’m guessing you would be able to look at a tent and instantly say if it’s going to work or not). You can get tents for a lot less than £700 that are returnable and probably come with extra things like carpets etc. One that expensive should be absolutely amazing!
If you’re going (and I would say it’s a HUGE “if”) and are not used to camping here are a few tips:
As you’re in a holiday park there should be plenty of facilities like an on-site shop, laundry (you’ll need the tumble drier to dry things once you’ve been in the rain 
). Check what’s there so know what to prepare for.
What has DH done about cooking and eating? Are you going to eat in the park restaurant or are you going to be doing all the cooking? If he’s expecting you to cook then take as much from home as you can. Don’t bother with buying “camping” pots etc as they’re useless and impossible to clean. Alright for hanging off a hillside and cleaning in a burn, useless for cooking for a family. Take oil, salt pepper, sauces, foil, chopping board, sharp knife, washing up bowl, liquid, more tea towels than you think necessary, kitchen roll and loo roll. If you’re cooking, how is that going to be managed? How old are the DC? You can’t have cooking inside a tent with DC and you need to have somewhere to out hot pans/serve up without people getting burned. It is really important that you think about this as it is potentially horrendously dangerous. You can’t have toddlers falling into hot pans or knocking into something that’s not safely placed.
Under no circumstances leave the house without quilts and pillows for everyone (maybe not for DH). None of this nonsense about “it’s not really camping”. You’re in a big tent on a park and there is no need to ever be cold when you sleep.
Take a hair drier. If you’ve got an electric hook-up you’ll be able to use it to thaw DC out when they come back in from the rain and you can use it to warm sleeping bags to get them cosy before bed. Depending on their ages, be prepared for them to fall asleep when it gets dark and wake up when it gets light (our normal is DC sleep 10pm and wake up around 6am. They only do this camping and we have invested in a blackout tent as we couldn’t stand it). You may get no sleep.
Buy a chair or something for you to sit on. I really like half height chairs. Tables are also handy as are rugs and the quilts and pillows you didn’t leave the house without.
Take waterproofs and suncream.
We love camping and go regularly. I would not even think about it if we didn’t both love it and weren’t well prepared. Good luck!