Hello, I am a solicitor with 20 years experience. I will try to help you with this.
You will have a right to quiet enjoyment of your flat, ie that you can exclude anyone including the landlord from entering without your permission in most circumstances. However it is reasonable for your landlord to show the flat to others who he wants to rent it to. he can not do this at any time. Late in the evening without your permission would be unreasonable and breach your right to quiet enjoyment. He should also let you know in advance when he wants to bring round viewing clients. Weekends are not in and of themselves unreasonable. Do you have young children who go to sleep before 9.00? You should contact the landlord and tell him when you are happy for potential clients to come round. If you have done this what has he said? Is there anything in the tenancy agreement covering this situation?