There is a sticky thread by Joa called:
Disrepairs in privately rented accommodation
I think the relevant paragraph is:
If negotiation fails, you could take one or more of the following courses of action:-
• use the rent to pay for repairs. Remember that this is a risky course of action, unless the correct procedure is carefully followed. You must not simply withhold rent
• take court action yourself
• get the local authority to take action
• if you are the tenant of a local authority or of a registered social landlord, claim compensation under the 'right to repair' scheme
• contact an ombudsman
• if the problem relates to electricity, gas or fire safety, contact the appropriate body
• consider eligibility for a housing grant, loan or other type of assistance for repairs or improvements.