Sorry for the delay in responding.
Check the original tenancy agreement in respect of viewings and whether there is an obligation for you to vacate. In all probability it will not state that you have to vacate.
If it does not state that you have to vacate and there has been no subsequent agreement that you must vacate then you are under no obligation to do so if you do not wish to.
In any event such a clause is very difficult to enforce even if it were there - what will they do, apply for a Court Order that you vacate, or attempt to sue you for a loss of income as a result of you not vacating upon visits, not likely at all.
If you do not agree then that may compel them to serve a notice to vacate upon you in accordance with what I assume is now a statutory periodic tenancy agreement, so that is something you should consider from a practical point of view. You may, for example, say that you will give them notice of when you are going to be out and say that they can arrange viewings then.
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