I'm in the middle of NOVAing, I tried to do the online form but couldn't find an option that sounded like my situation (private purchase abroad without going through a professional dealer or importer, and without originally the intention to import at all). It was difficult but I finally managed to get through on the phone, the chap was super helpful and sent me a paper version of the form. He said just explain the situation in writing on the back of the form and they would work it out. There won't be any import duty to pay because of the 10 year rule.
Do note the important technicality that NOVA has to be submitted 10 working days after arrival to avoid a fine, or failing that 10 days after your "change of intention" which is simply when you decided to keep it in the UK. This rule is explicitly to avoid penalising owners of foreign registered cars that have been in the UK for years, undergoing restoration etc.