The notice is not valid for the purpose of s21 Housing Act 1988, which means you are not under notice. Practically this is of little value to you since you are moving out before the expiry of the notice period anyway. It does, I suppose, afford you some security if your alternative accommodation falls through since they could not apply for an order for possession from the Court based on the notice they have served.
For the purpose of the clause relating to viewings the "notice'" you have received may be held to be a valid notice for the purpose of this clause even though it is not a valid notice for the purpose of s21 Housing Act, strange as it sounds.
I'm not sure what kind of proof you would ask for to determine that the viewers are genuine, if you can think of any then it is a reasonable request I would think.
If you do not allow viewings then you are in breach of contract and although it's doubtful that they would seek to claim possession on these grounds or sue you for any loss suffered (because such loss would be difficult to prove) you never know because you have doubtless irked them by quite rightly issuing proceedings against them for the deposit.
I would be strict with them on the hours, times and lengths of visits that I would permit but I would permit them.
Hope this clarifies, if so please click accept.