They can send you a cheque all they want, all you need to do is throw it in the bin and call & email them confirming that you are not accepting their settlement figure at this time. Where is the car right now? If it's in a yard waiting to be scrapped, they're paying storage costs so want it shifted ASAP, hence them trying to push you. If it's on your drive, I'd be trying to secure it best I could.
Am I reading this correctly that THEY sent YOU some adverts, 3/4 of which were above their settlement offer? If that's the case, then I would send them an email stating that even their adverts are proving your case. Unless your car was covered for the value of the mods, then you have to value it as a standard car I'm afraid.
Do me a favour, PM me with the amounts we're talking about here and the company involved, I can probably help you more that way. Only if you want to, of course.