Look at the CAB website:
http://www.adviceguide.org.uk/index/family_par ent/housing/disrepair_in_rented_accommodation. htm
under this heading: Local authorities� duties.
I had the same problem in a rented accomodation in 2005. We had found the property through an estate agency so they were able to advise us on what to do. We had moved in in the sept and there was definatley no damp - by the nov it was so bad you could smell it when you walked in the front door.
we notified the landlords but they kept coming up with excuses as to what could be causing it. they even told us it was caused by the condensation of steam after cooking!!!!
the landlords eventually fixed the problem after we threatened to notify the local authorities.
it's very important to keep a detailed record of all phone calls and conversations as well as letters written to your landlord. make sure you write down date, time, who you spoke to and summary of what was said - just incase your landlors don't comply.
it's definatley their responsability to fix - contact your CAB office and get some advise on how you should start addressing this problem.
on the plus side... it's moving into summer so hopefully the damp won't get worse before they fix it.