If you talk to the main guy at Eve Brown in person (Stephen Brown) then that usually sorts out the problem. He can then take it up with the landlords.
Talking to the little people at an estate agents will get you nowhere whereas if you speak to the top guy then they know that you know where you stand.
But really you should have taken photo's and noted down everything on the original inventory before you did anything to the place and then take more photo's of it when you leave to show how the conditions compare.
Without photographic evidence you would have no chance in a small claims court which both the agents and the landlords will know fine well.
On top of all this when moving into a flat you should consider the deposit non returnable in the first place - you must be prepared to lose it. For a short term tenant (student or otherwise) it is always a bonus rather than a given to get the deposit back (believe me I know!!).
But talk to Mr Brown and see what he says, he's actually a surprisingly nice and non satan worshipping bloke