How can I get my deposit refunded.

Tenancy Deposits

My daughter shared a flat with two other students.July 08-June 09. The tenancy agreement was AST for 12 months and obligations under this agreement were joint and several liability. Each tenant had a signed guarantor.
The property was vacated at the end of June 09 (deposits are protected by TDSL) . To date deposits have not been refunded. I have been in contact with the Landlord who states that one of the tenants owes rent. He will not disclose how much is owed, even though he is withholding refund of the deposits because of this. The person who owes the money is overseas at present. The Landlord says he has written to the tenant owing the money and her father (Guarantor) requesting payment. My question is, if this back rent (how much the landlord will not disclose because of data protection) is not paid, what action can I take for the return of my deposit.