Garage wanting double what they quoted or no car back

[Fullyhuman]

Hi,
Can anyone tell me, where do I stand? A body shop has my car, which i crashed back in April, he quoted £3k and now wants £6k with no justification - he cites inflation! And says if I don’t pay I can’t have my car back. My husband thinks a solicitor would be good money after bad and that the police won’t care. I don’t know what to do. The car isn’t worth loads more than £6k, maybe 8.
thanks

he can’t sell it can he? I have the owner certificate.

I’m certain he can hold it under a mechanic’s lien until payment is made but I’m not sure if he’s able to sell it after a certain period. Not sure what an “owner certificate” is but there’s no such thing in the UK.