bednobs is incorrect.
Under a HP agreement, the credit company is jointly liable for the car under section 75 of the Consumer Credit Act.
If the garage do not want to know and are being unhelpful, go straight to the finance company and tell them that you have issues with the vehicle under section 75 and wish them to correct it and that you are suspending payments until they have sorted out the problem.
This is only on the basis that you have a Hire Purchase agreement (as stated in the title) and not a loan agreement (check your agreement, it will usually state this on the top)