Voulentry Withdrawal

[Lpl87]

Hi all, looking for some advice please. I bought a used nissan qashqai from Evans Halshaw 18 months ago. It is literally falling apart I pay extra on my finance agreement each month for a guarantee in which I was told covers everything apart from serviceable items. LIES!! Sent it in with a few issues had to pay them £50 first for them to tell me what was wrong with it although I already knew that's why I took it back there for them to fix it under the apparent guarantee. They have never given me any sort of list either on what is and isn't covered. They got back to me saying o my a couple of things are covered so that I would get my £50 back. Took them 3 months to refund me! Just failed an mot on the issues I wanted them to fix that has been there since I first bought it. I have had enough now and want to withdraw from the contract. I understand that I will have to pay half of the balance which infuriates me! I have read on here that it should be in good condition for this to happen but it clearly wasn't when I bought it. Where do I stand? Would be grateful for any advise please.