I worked for Welcome Finance for several yrs and came across several people who had been mis-sold their cars or the cars were not road-worthy.
I know for a FACT that if you persist with your complaint and really threaten to come and dump the car at their office/refuse to make payments they will act. To be honest we were always told to threaten legal action despite the fact in the 5 yrs or so i worked for them they never once took anyone to court - simply because of the high interest rates they didnt want the bad publicity. The longer you hold onto the car the harder it will be to return the car and more liability you are likely to incur. Best bet is take it back and dump at their doorsteps with the keys with a letter telling them your position. Dont let them palm you off with threats - they are just empty threats of action.
Demand to speak to the branch manager of Welcome and tell them you want to return the car due to the fact it was mis-sold under the Sales Act.
Im not sure you will get a refund but im sure they will - IF you persist write off the loan and take the car back.
Finally my recommendation to anyone reading would be DO NOT buy from Welcome Finance. My own experience was they simply dont care once you sign on the dotted line. DO NOT take out their insurances they are utterly useless and can be found much cheaper elsewhere. You end up paying a high interest rate on-top of an already useless insurance policy.
Do speak to a Solicitor regarding your rights or even trading standards should be able to help. I look back with great regret that in my time working there i saw so much done wrong yet didnt do anything about it. They are vermin with little care for anything or anyone bar their targets and manipulating the public.