Why do you think Modelsport owe you anything? They have done the right thing in offering you a refund, if you want it that is, and that is fully your choice, if not keep the product and move on.
The example of buying a TT01 and receiving a chassis is no different, you would complain and return the goods for a full refund, OK it is annoying but it happens, if the seller doesn't have a full TT01 you cannot make them supply you with one.
The law states that you should be in the same state before the error, in other words you should be paid back the same amount that was paid to the company in the first instance (I don't think this includes postage). The law does not deal in compensation and rightly so, you should not be entitled to more than you had to begin with.
Morally Modelsport do have the option to offer you extras if they want to keep you as a loyal customer however that is not your "right" and will depend on the company and individual.
In the past I have had extras thrown in due to waiting a long time for parts, they were not to blame but were happy to help, on another occasion I had parts on an excessively long wait and asked for a refund, they were happy yo oblige and I didn't get any extras, why would I?