In reply to mypyrex:
basically they are invoicing you for works that were previously carried out,
however they did not get authorisation to carry out the works.
I would ask them to provide the authorisation (which they cannot)
If they attempt to make an additional charge on your card ask your bank for a charge-back as the monies have not been authorised by you.
If they insist and take you to the small claims court,
they did not have your authorisation to carry-out the repairs to that value.
they should have taken the payment before or as you collected your property.
they do not have a leg to stand on