In the first instance check what it says in your Tenancy Agreement as to what you agreed regarding landlord access to the property. Beyond that the key pieces of legislation that cover landlord access are as follows:

Landlord and Tenant Act 1985 S9A (7) & (8) – There is an implied covenant by the lessee that the lessor, or a person authorised in writing by the lessor, may enter the dwelling for the purpose of viewing its condition and state of repair. to the dwelling to be permitted only at reasonable times of the day and only if at least 24 hours’ notice in writing has been given to the occupier of the dwelling.

Landlord and Tenant Act 1985 S11 (6) Repairing obligations – In a lease in which the lessor’s repairing covenant is implied there is also implied a covenant by the lessee that the lessor, or any person authorised by him in writing, may at reasonable times of the day and on giving 24 hours’ notice in writing to the occupier, enter the premises comprised in the lease for the purpose of viewing their condition and state of repair.

Housing Act 1988 S16 It shall be an implied term of every assured tenancy that the tenant shall afford to the landlord access to the dwelling-house let on the tenancy and all reasonable facilities for executing therein any repairs which the landlord is entitled to execute.

Have you had problems accessing the property or problems with your landlord just turning up unannounced?