A landlord, or his agent, or someone authorised to act on his behalf has a right to view the property to assess its condition and to carry out necessary repairs or maintenance at reasonable times of the day.
The law says that a landlord or agent must give a tenant at least 24 hours prior notice in writing (except in an emergency) of such a visit. Naturally, if the tenant agrees, on specific or odd occasions to allow access without the 24 hours prior written notice, that is acceptable.
A clause in the tenancy agreement which tries to diminish or over-ride a tenant�s rights in this respect would be void and unenforceable.