Section 11 of the Landlord and Tenant Act 1985 places specified repair duties on landlords for the tenancies within its scope, including the ordinary private assured tenancy covered here. A clause cannot simply transfer those statutory duties to the tenant.
The duty concerns particular parts of the home and installations. It is not a guarantee that every supplied object is covered by this section or that every fault must be fixed within one universal number of days. Identify the problem, report it and check the relevant responsibility.