As Cath says, the deposit should have been held in a deposit protection scheme, & you should have had in writing from the agent or landlord details of which scheme. The scheme is responsible for looking into any dispute between you & the landlord about return of the deposit, so contact them.
To me, it seems entirely unreasonable for the landlord to keep half the deposit just because your ex is uncontactable. I cannot see why they should refuse to return it all, especially if you give them an indemnity - but ask the deposit protection scheme. It looks very like a one sided arrangement, as either of you would be fully responsible for the rent payments (i.e. joint & several liability).