There is a statute which allows a tenant in specific circumstances when a tenant can deduct or set off sums from rental payments when it has incurred sums to remedy defaults by the landlord in computing with its repairing obligations.
However assured shorthold tenancy agreements usually probit the tenant from exercising this statutory right.
If you do deduct from rent the landlord can recover the sums out of the deposit and you will have to top up the deposit. Or the landlord can take it as a default on payment of Rent, it could be recovered from you via the court with interest and fees added.
You can undertake the works and request the landlord refund you, if it fails to do so then you can take the matter to the small claims court to recover the amount, plus interest and court fees.
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