Short answer = landlord

Long answer….

1. The Landlord and Tenant Act 1985 section 11, which is implied into all tenancies of less than seven years duration, prescribes that the landlord must keep in repair and proper working order the installations for the supply of water including basins, sinks, baths and sanitary conveniences – a tap is part of the installation for the supply of water.

2. In Warren v Keen (1953) Lord Denning LJ stated ‘he (the tenant) must do the little jobs about the place which a reasonable tenant would do. In addition, he must, of course, not damage the house, wilfully or negligently; and he must see that his family and guests do not damage it; and if they do, he must repair it. But apart from such things, if the house falls into disrepair through fair wear and tear or lapse of time or for any reason not caused by him, then the tenant is not liable to repair it’ Using a tap repeatedly will inevitably lead to some wear and tear and with the lapse of time leak. Wear and tear is what tenants pay rent for.

3. Nowadays, the vast majority of modern taps use cartridges (typically ceramic disc) instead of rubber or leather washers. Would you trust a tenant to repair the installations in your property?

Time to get out your monkey wrench!