Can I refuse a viewing if the landlord is selling or re-letting?Beta*

Even if your agreement says you will allow viewings, the landlord can't force their way in. Entering after you have said no would breach your right to quiet enjoyment. It's usually best to offer other times that suit you, in writing, so you can show you haven't been unreasonable. If you are ill, or the request is at short notice, you can say no to that slot. A landlord who keeps pressing, threatens you, or comes in anyway should be reported to the council's tenancy relations officer.

Sources: Shelter: landlord access and viewings