Return rent deposit

Rent Arrears

I manage 1 of my properties myself. Current tenants have been in the property since Feb 2010. I want to sell and sent an email and letter in August asking them to leave with 2 months notice, which they indicated by email they agreed to do.
Unfortunately I never placed their deposit (1 month's rent) with a protection scheme. I now realize my mistake and have been trying to return the deposit by bank transfer but despite my attempts to find out the tenants' bank details they are not responding. I understand that if they do not leave voluntarily this will count against me if I apply for a court eviction. My question is, how can I return their deposit (so that I am not holding a deposit I shouldn't be) if they won't accept my payment?