Short answer – as stipulated in the tenancy agreement. Where the tenancy agreement is silent on the matter, then neither party (landlord or tenant) is legally liable for the repair.
Long answer …
Section 11 of the Landlord and Tenant Act 1985 inserts into all (qualifying) tenancy contracts an implied covenant for the landlord to keep in repair and proper working order the installations in the Dwelling-house for the supply of water, gas and electricity and for sanitation (including basins, sinks, baths and sanitary conveniences, but not other fixtures, fittings and appliances for making use of the supply of water, gas or electricity).
A 1996 report by the Law Commission (LANDLORD AND TENANT: RESPONSIBILITY FOR STATE AND CONDITION OF PROPERTY) stated ‘In general, the landlord will be contractually liable to the tenant only where he has expressly undertaken an obligation to repair or maintain the property. “It is well established that, in the absence of agreement to the contrary, the law imposes no obligations on a landlord to keep the demised premises in repair.” citing Sleather v Lambeth Borough Council 
Elsewhere, the report states: ‘There is a presumption against the implication of any repairing obligation in a lease; there may therefore be cases where neither party is obliged to repair all or some part of the premises that are leased.’
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