one of my colleagues reckoned that technically it was joint liability
A good example showing why you should never listen to pseudo-legal bollocks.
Surely the Co. can't issue you with a card in your name? you should have to apply for it directly to the issuing bank?
The company requests the bank to issue the card in your name, and is fully liable for payment. However, the company should have stringent and enforced rules for use of the card, which holders must sign up to. One I always make and enforce is that any payments made on the card which are not accounted for by proper receipts for legitimate expenses are eventually deducted from wages and treated as salary, ie subject to PAYE and NIC (in the UK), which happens also to be the HMRC rule, boiled down to its essentials. Any decent properly-funded operator will have a fuel carnet or equivalent.