UK Property Law
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It is not legal for you to withhold rent from the Landlord because of the disrepair. For you to do so would mean that you are in breach of contract and therefore the landlord could serve a notice to vacate on you.
As to repair, the Landlord is under an obligation to keep the property in a good state repair and maintenance. If it is in disrepair, request that he carry out the specific works within a reasonable time (say 14 days) in a letter to him. Also state that that if he does not then you will pay to have the works done and seek to claim the expense from him
If he does not carry out carry out the repair works you can pay to have them done yourself. You can then claim the money back from your landlord by issuing a claim through www.moneyclaim.gov.uk
You may also consider contacting the environmental department of your local authority. They will send an officer out to inspect the property, if he finds it uninhabitable then you will be able to terminate the agreement with your landlord.
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