Thanks for replies and advice.
I did actually send an email to the ceo of Hastings yesterday afternoon and he got a senior manager to telephone me this morning to discuss my problems. He was very amicable and he agreed to pay another £350, so £1700 which is close enough to the £1900 I originally paid for the car. So I've agreed to settle.
So in this case contacting the ceo (and politely) did work. As you have said the fos probably wouldn't have helped at all and going to the small claims court for this small amount wouldn't have been worth it.