This is messy. First we need to establish whether there was a tenancy; in my opinion there was. You do not need to actually sign an agreement to create a tenancy. You paid for an exclusive possession, i.e. for a sole right to occupy this property and therefore your and the landlord's actions created a tenancy.
You have left early without the landlord's agreement. Therefore you are liable for the remainder of the rent, unless the landlord gets someone else in. You are therefore responsible for 6 months worth of rent. If you sue for the deposit plus 3 times the amount, guess what the landlord is going to counterclaim for?
My opinion is; watch very carefully if some new tenant moves in. This will mean that the landlord has "given up" your tenancy (surrendered) and you are no longer liable for the rent payments.
Then you can start the deposit recovery action.
Alternatively, you can take the risk that the landlord is going to be intimidated by the penalty for not entering into a TDS and pays the deposit back. You may play the benevolent, generous tenant who allows the landlord to give you back your deposit, instead of dragging him through the courts. But if the landlord is clued up or seeks legal advice- the game's up mate.
Lesson to us all; check, double check and triple check before moving in.
I will be happy to stand corrected if any of my learned friends wish to do so.
There, how posh
landlord witholding deposit 
