Aaron
If you solicitor has advised you that the contract with Welcome is illegal, DO NOTHING. Do not even speak to them.
Let your solicitor write to Welcome to draw to their attention to the flaws in the contract.
Speak to your solicitor about whether you should stop payments. My opinion is that you should continue to pay while this matter is being sorted out by your solicitor. If you stop payments, you will receive even more harassment, with visits by doorstep collectors. And Welcome could take action to reposess the car and sell it. You could then owe them a lagre some of money and have defaults placed on your credit record.
Huckster