Here is a simple scenario you might adopt:
1. If you didn't have any other enquiries for the Easter period after the booking was taken, you are at no loss; so send it back.
2. If you turned away other enquiries, you are at a loss; Tell the people that you had to turn away other people, and that you will try to re-let for the Easter; Tell them that if you succeed in re-letting, you will refund their booking deposit(mius a small admin fee if you deem it appropriate). If they are reasonable people, they will accept this. In the absence of a written contract, the implied terms of your agreement should be reasonable; I think that a court would find the above reasonable.
3. For the future, go to a large and professional holiday home website (or a few different ones), read their terms and conditions, adapt them to meet your needs as you see fit and then go to a solicitor (if you feel the need) to give the thing a final once-over.