Thanks for your question.
If the state of repair (particularly damp) is very bad then you should contact the local authority environmental health department and ask them to send an officer round. If the state of the property is bad he will issue an enforcement notice against the Landlord to carry out works and this should prompt them in to making the repair. If he finds it unfit for human habitation then he will declare it as such and you will be able to terminate your agreement with the landlord on the basis of this breach.
The Damp is, in effect, a disrepair issue. You have a right to a property in repair and can enforce this against the landlord.
You should formally write to the the landlord (copying in the agent) specifying the disrepair, making a list of the reasonable repair required and ask that he make those repairs within a reasonable time (eg. 14 days). State that if the landlord does not make the repair within that time you will pay to have the repairs made and will seek to claim the expense from him and are prepared to make an application to court if necessary.
If the landlord does not make the repair you can pay to have it done and then write formally to him requesting the payment of the cost, again within a reasonable time). If he does not pay you can issue a claim for the money yourself through Her Majesty Courts Service's online service: http://www.moneyclaim.gov.uk/. Write to them first asking for the money and stating that you will issue a claim if it is not received.
Its pretty cheap and straightforward to use.
Whilst the damps is still present and preventing use of some part of the property (if it is) then you can ask that the rent be reduced and you may choose to negotiate on this point.
Open a file, send letters by registered post, keep copies of everything for use in any claim you may choose to issue at a later date.
Once you make efforts to formalise things landlords tend to be more compliant.
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