My question is, is there a limit to how many times over the finance companies can claim interest? as my parents have probably paid the original amount 10 times over in interest alone!
No there isn't, tell them to clear the balance in full and end the agreement......did you mean £150 or is that a typo?
Otherwise if they take a further 20 years to clear it...they could end up paying the original amount 20 times over in interest alone.!.