If it was a normal property transaction, you would be able to submit a 'Notice to Complete' to the vendors, as technically completion has not taken place. As CAS said, you can contact a locksmith and gain entry that way, and then submit a bill to the vendor's solicitors, but I would advise that you get your solicitor to make a call first to get authorisation. I can't remember the procedure regarding repossessions, but there is bound to be some sort of legal remedy.