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Under an assured shorthold tenancy you cannot withhold rent from your landlord. If you do you will be in breach of contract and run the risk of of him serving you notice to vacate. I do thought sympathise with your position.
You could serve a notice of repair by letter on your Landlord requesting that he make the appropriate repair within a reasonable time (usually 14 days) stating that if he does not then you will make the repair yourself and seek to claim the expense from him. If he does not make the repairs within that reasonable time , then you could pay to have it done yourself and then make a claim for the money from your landlord through: www.moneyclaim.gov.uk
The problem appears to be the length of time it would take to make the repair. If you just want out of the property then you should phone your local authority's environmental health department and ask them to inspect the property. The will send an officer round and if he finds that it is uninhabitable then you should request that he confirm this in writing to you.
When you receive this written confirmation you should write to your Landlord enclosing a copy and state that you are terminating the agreement because of his breach. This breach is a repudiatory breach which would entitle you to terminate the Contract.
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