Did they pay over a deposit on exchange of contracts? If so, what percentage of the purchase price did they pay over?
Has this been transferred to you?
So you exchanged last year, was the completion date fixed or was there a longstop date as your question refers to monthly penalties, which would assume the contractual completion date had passed, or a longstop date or similar?
Has any notice to complete been served as I note that you say that the contract expires in December.
Normal notices to complete give around 10 working days (depending what was in the contract) for completion to take place or the seller can rescind (walk away from the contract) and keep the deposit.
There is usually an entitlement for a 10% deposit allowing the extra to be claimed should they have paid a lesser sum on completion.
If you go after them for breach of contract then the remedies available if successful are described here...
http://www.wheelerslaw.co.uk/site/library/comm ercialgeneral/remediesforbreachofcontract.html
Some info about enforcing court orders relating to money...
http://www.hmcourts-service.gov.uk/infoabout/e nforcement/index.htm