A little update:
We went back to the old house because my housemate left his chest of draws there so we wanted to pick it up. We also thought we'll have a look at the house and see if the land lord has actually done anything.
Turns out the MFer had sold the house. There are new students living in there now, and the landlord happens to be one of the student's father. We spoke to them, took pictures of the "damages" he listed. We have the backing of the new residents and in turn the new landlord. The students met the old landlord and didn't think much of him at all.
We also went to the estate agents where the house was advertised almost 2 years ago because one of the charges were for not paying our retainers (£165 each [also mentioned in contract], landlord wrote £175 not payed from 4 of us). They had a record of us giving them the cheques. So now that's not our problem.
We're going to write a polite letter to him, advised by the student services (who also happened to mention that this landlord was notorious), giving our reasons why we aren't paying him anything, and see what he'll do.
Edit: I forget to mention in the OP. The contract for the rent was until 31/7/07. We vacated it on the 15/7/07 without a choice because the landlord and his workers had bagged up all of our stuff ready to be thrown out. He immediately proceeded to change the locks, which AFAIK is illegal?