Within 6 months of purchase, the onus is on the seller to prove the product si fit for Purpose, meaning that if it went to court you'd have a massive chance of winning.
- Get another garage to inspect it
- Get onto your dealers with the fault
- If they don't play ball, issue a Letter before Action
- If they still don't play ball, issue a claim through the Small Claims Court
- You win, and get the cost of the repair, compensation and costs back.
This is why you should ALWAYS pay for a car on a credit card when you can. A guy on the MG Rover forum bought a 2004 Rover 45. It had random electrical issues 3 months from buying, dealer said sod off, and in the end the credit card company reversed the payment.
Keep fighting, you'll win I reckon. Perseverance.
