Utter joke mate, but tbh the first offer always is, however normally it's not as vast as that difference.
Please say you have receipts for the mods and the work hours charged for fitting them.
I appreciate the situation but I'm really thinking you should have told them to jog on when they wanted to collect the car, wether insurance ran out or not is irrelevant as the claim was whilst the policy was active. Youve made sure the renewal hasn't gone thru yeah? I'd not be going with this insurance company again.
To prove market value, cant you stick the car up on eBay as it was with a BIN/best offer on and see what it goes for?
I know when my ibiza died they were comparing ads on ebay, auto trader and they even accepted stuff I mentioned from pistonheads.