Thanks for the advice, everyone. Motability has put in a case manager, and I’ll be speaking with them next week.
Tbh, the whole situation has put a sour taste in my mouth. As others have mentioned, I realistically don’t want to deal with this dealer at all, and all the excitement of a new car has disappeared for me. Also, one of the main reasons I chose ID.4 was the price, as the £1000 advance payment was just something I could afford, and it was an enticing deal at the time.