Lighten up, the advice on here is not Gospel, half of it is plain wrong the other half should be taken with a large pinch of salt.
Admit the snide comment did not help, but your £2.00 an hour statement wasn't exactly useful.
Still think you are wrong with the keys though, I would always return the keys when asked even if owed a lot of money, I would argue that by doing everything by the book would strengthen the case if it ever came to court, also if anything should happen to the company while you are holding the keys you would become suspect number one.
There is a reason the contract was lost, that could also be the reason for non-payment. There should be a procedure in place and there is a contract signed.
The contract is gone, if it is believed that a good job has been done then payment is due, so go and see them, if they get huffy then bring the contract into play and charge another month. If you have nothing else to do it could be fun!!