See a solicitor.
Best advice right there and probably really the only decent course of action when you've got a dodgy landlord.
A good friend of mine back when I was at Uni was in a pretty rotten situation with a landlord. He was in a bad way as it was, as his Father had just died, but the place he moved into had no working cooker/oven, all the chairs in the house which had been included as part of the rent before they moved in had been removed, the water didn't work in the bathroom sinks, the boiler was broken, so they had no hot water and there were numerous other things wrong with the place.
They first went to the landlord about it and he did nothing. So then they said they wouldn't pay rent that month if he didn't fix the problems still. At that point, he then threatened to take legal action if they didn't pay their rent. I guess there are also a lot of Landlords who think students are easy pickings, though I guess things have changed a bit in the last few years to help protect tenants a bit better.
Anyways, my friend, who was pretty sick of it, went to a Solicitor who drew up an Ultimatum and to cut to the chase, the landlord was forced to fix the problems straight away, give back three months rent and pay them £400 each in compensation. So it worked out quite well and gave the landlord a firm kick up the arse.