Ask an Property Solicitor. Get an Answer ASAP.
My name is ***** ***** I will try and help you today.
I am sorry you are having problems with your landlord/agents.
Ultimately, you have a right to refuse entry as the property is rented to you and the landlord/agent owes you a covenant of quiet enjoyment.
This means you cannot be hassled by the landlord whenever they require access.
The landlord should also provide notice of an intended visit usually 24 hours unless it is an emergency.
The bot***** *****ne is you should try and negotiate with them to reach a mutually convenient arrangement so they can have access when it is convenient to you.
Check the tenancy agreement to see if it says anything about viewings otherwise you can refuse entry.
Basically, but there may be a specific clause in it about viewings.
In any event you can refuse entry.
You are welcome.
Best of luck!
Although, technically you can refuse outright you should allow the landlord the opportunity to show new prospective tenants around and be reasonable about it.