You are entitled to request one of 4 remedies, repair, replacement, full refund or partial refund, if repair has been tried and failed then it strengthens your case for one of the other options. If you paid for it on finance then contact the finance company as they have equal liability for any problems, similarly if you paid at least some on a credit card try the CC co.
The garage seem to be offering the lowest amount of assistance they can to scrape within the law so they are not doing anything technically wrong by just offering repairs, but you have certainly got a strong argument for other options.