WHAT ARE MY CONSUMER RIGHTS? DOES COMMON LAW PROTECT ME IN THIS CASE?

Five (5) weeks (25-02-2025 to 29-03-2025) after purchasing a 2nd hand car the thermostat needed replacing (done by my regular Workshop) for which Weelee agreed to pay in a very time-consuming process. In the 5 weeks I had use of the car, it needed two and a half liters of oil & a qualified mechanic pointed out that the car engine had been opened before & sealed with silicone. The final diagnosis was that the head gasket had blown. Numerous discussions/phone calls later, I received a request from my Warranty consultant - they needed a brief description of what went wrong with the car before they could process a claim submitted on my behalf (?); Weelee's workshop (where the car had been taken to) had submitted a claim for the repair of the head gasket PLUS the already paid (to my workshop) thermostat without consulting me. To my mind the repairs should not be claimed against my warranty - I had been very clear about cancelling the deal for a defective car sold to me. I sought legal counsel - a letter of cancellation and demand was issued. No response from Weelee within one week. What do I do now? Enter into a mediation process as advised by legal counsel?