"As for withholding the rent, Leaders note that "a tenant does have the legal right of off-set" ? basically, you have the right to hire someone to do work which is the landlord's obligation and then withhold the costs from the rent.
However, withholding rent is risky and you can be evicted if rent arrears build up. There is a precise process for tenants to follow with this, which Citizens Advice explains as follows, but they warn that you should not attempt it without first talking to one of their housing advisors or a solicitor.
? Give the landlord notice of the disrepair and a reasonable time to remedy it
? Inform the landlord (preferably in writing) that s/he will do the repair her/himself unless the landlord complies with her/his obligations
? Allow a further reasonable period for the landlord to do the work
? Obtain three estimates for the cost of the work from reputable builders
? Write to the landlord again, enclosing copies of the estimates and reminding her/him of her/his obligation to do the work, giving a further reasonable period to carry it out. The letter should warn that, otherwise, the tenant will do the work her/himself and deduct the cost from rent
? If there is no response, arrange for the contractor who gave the lowest estimate to do the work, and obtain (and send to the landlord) receipts, with a request for payment
? If the landlord does not pay, the tenant may deduct the cost from the rent (but not other charges such as service charges), then send the landlord a breakdown of the amount and period of the rent to be withheld.
The tenant, they add, should avoid running into rent arrears before taking this action. If, however, the landlord takes court action for arrears and/or possession, the fact that the tenant has had to pay for repairs can be used as a counterclaim to the proceedings.
Damages caused by the landlord's default can also be set off against arrears in a defence to possession proceedings. Specialist advice will be needed.
Shelter have a useful page on this subject as well, complete with handy sample letters to send to your landlord. "
Also You and Yours on Radio 4 at the moment is all about dodgy landlords, might be worth a listen.