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Check the tenancy agreement for the clause relating to repair. It will likely say that it is the Landlord shall keep the property in repair. If so you should write to the Landlord formally requesting that he make the specific repair to the boiler within a reasonable time (most people state 14 days). Also state that if he does not complete the works then you will pay to have them done yourself, seek to claim the expense from him and are prepared to make an application at Court for the money if necessary.
If he does not make the repair then you can pay to have the works done yourself and make a claim yourself through www.moneyclaim.gov.uk.
You should not withhold rent from your Landlord, if you do you will be breach of contract and he may become entitled to serve a notice to vacate upon you.
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