OnZephyrstdayofXmas, disrepair and with holding rent has come up recently and I apologise but I've cut and paste(d) my previous response:
Firstly it depends on your tenancy agreement. Protected tenants and local authority and registered social landlords tenants have the right to carry out improvements themselves, with their landlord's written consent.
If negotiation fails the tenant could take one or more of the following courses of action:-
You can use the rent to pay for repairs. BUT remember that this is a risky course of action, unless the correct procedure is carefully followed. The tenant must not simply withhold rent
to use rent to pay for repairs, or to offset the cost of repairs against arrears, your sister must carefully follow (in order) the steps below:-
give the landlord notice of the disrepair and a reasonable time to remedy it; then
inform the landlord (preferably in writing) that she will do the repair her unless the landlord complies with her/his obligations; then
allow a further reasonable period for the landlord to do the work; then
obtain three estimates for the cost of the work from reputable builders; then
write to the landlord again, enclosing copies of the estimates and reminding him of 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 herself and deduct the cost from rent; then, 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; then
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.
As you can see this is a lengthy process and it may be in your sisters interest to simply start negotiating with her landlord to get the work he has agreed needs doing done sooner.
Beyond this comes
take court action
get the local authority to take action
contact the local government or independent housing ombudsman
. HTH & goodluck