Many thanks for your patience. When a consumer makes a purchase from a business seller, they will have certain 'statutory' rights under the Sale of Goods Act 1979 and the Sale and Supply of Goods to Consumers Regulations 2002.
The law states that the goods must be of satisfactory quality, as described and fit for purpose. If they are not, you will have certain legal remedies against the seller. The only time action can be taken against the manufacturer is under a manufacturer's warranty or guarantee. There is no protection against fair wear and tear, misuse or accidental damage, faults that were pointed out at the time of sale or if you change your mind and no longer want the goods.
So if you have a problem, which now may even be outside of the warranty period, you still have rights under statute. You can also argue that the repairs they carried out were incorrect and unnecessary and you were misled into having to pay these costs which have in no way resolved the issues. As such you should be entitled to still have the vehicle fixed, although they my still retain a proportion of the repair costs to fix the actual problem (you should nevertheless be refunded for the unnecessary work carried out).
You can quote the applicable laws and rules as mentioned above. If they appear reluctant to assist, write to them one more time, warn them that they have 7 days to comply and inform them that if they fail to meet their legal obligations, you will have no choice but to report them to Trading Standards and start legal proceedings to seek compensation for your losses.
This is your basic legal position. I have more detailed advice for you in terms of the steps you need to follow should more formal action be required, which I wish to discuss so please take a second to leave a positive rating for the service so far (by selecting 3, 4 or 5 stars) and I can continue with that and answer any further questions you may have. Don’t worry, there I no extra cost and leaving a rating will not close the question and we can continue this discussion. Thank you