They are liable to pay the full rent due under the contract unless the Landlord agrees otherwise or there is a clause in the contract which provides for a rent get out, I seriously doubt it though as much as the Landlord could get out of paying their mortgage just because there is no rent coming in.
Was a deposit paid which, provided there is nothing the Landlord could keep it or any part for, could, with the Landlord's agreement, be put towards part of the rent.
The Landlord would be entitled to go after any outstanding monies in the courts and if there is a valid contract in place then I can't see them being unsuccessful.
Court action and the potential ramifications should be avoided if at all possible due to future credit rating problems and if they want to rent another flat at some point.
So it may be worth coming to some kind of repayment agreement for the amount owed, factoring in if the Landlord manages to get another tenant to take over the rent.