Illustrative example
A landlord needs to get a gas engineer in for the annual safety check. Two appointments have already been missed by the landlord's own contractors, and the tenant has now stopped agreeing dates.
Section 16 implies a term that the tenant shall afford access and all reasonable facilities for executing repairs the landlord is entitled to execute, so a flat refusal puts the tenant in breach of a tenancy term. The fact that decides how reasonable each side is being, though, is the history of missed appointments: the duty is to afford reasonable facilities, not to sit at home indefinitely, and the twenty-four hours' written notice requirement comes from the Landlord and Tenant Act 1985 rather than from this section.
The tenant offers three dates in the next fortnight and the landlord agrees to confirm one in writing at least a week ahead, with the tenant free to refuse any visit not confirmed that way.