Under contract law a party's failure to act within an agreed time limit constitutes a breach of contract. So if the agreement was that the bathroom was to be finished by Christmas and you agreed, then failure to complete the job by that deadline amounts to a breach of contract. The customer is entitled to seek compensation for the breach. If the sum of £2,000 was discussed then I'm afraid the customer is entitled to seek that amount although you might be able to agree a smaller sum to avoid litigation. The botXXXXX XXXXXne is, however, that when a deadline is missed, the party which has been disadvantaged is entitled to seek compensation. Hope this clarifies the matter.
Happy to discuss further if needed. Ale