Landlords and/or their authorised agents may request access at reasonable intervals to carry out repairs or inspections of the property. This must be done at a date/time agreed with you in advance. If a suggested time is not convenient, an alternative should be arranged as soon as possible. There is no legal minimum period of notice that must be given (e.g. 24 hours). It is a matter of what is agreed between you and your landlord/agent.
Unless there is an emergency situation (e.g. fire or flood), a landlord should not call to the property unannounced or enter the property without your permission.
If renting on a room-by-room basis the landlord may access the common areas without consent and may show vacant rooms to prospective tenants.
A tenant cannot change the locks without the landlord’s consent and the landlord would be entitled to a key for emergencies.