To be valid a s21 notice must give two months notice with such notice to expire at the end of a rent period. The end of the rent period in the tenancy agreement is 11th of the month so if this were accepted to be the end of the rent period then the notice is invalid on this basis.
If however it was accepted by the Court that both parties had agreed to the variation of the rent period to start from the 1st of each month then the end of the rent period would be the final day of the month. Provided you gave two months notice to expire on the final day of the month then the notice would be valid. However, if the tenant disputed this variation then it would be up to the judge to decide whose testimony is more credible.
There is another problem though. If you left the notice outside her door then this is not a valid form of serving the notice because you cannot guarantee that she got it unless she confirms this to you and the Court. If she confirms this then there is a chance the Court might state that the notice is valid and you were entitled to possession.
If she states that she did not receive the notice outside her door then you would instead by relying on the notice that you sent in the post, but this is not valid because the notice period only runs from the day they receive the notice and you have to allow for posting (which you did not do).
There is a serious question mark over whether the notice is valid I'm afraid.
If the notice was invalid then the tenant will be able to sue you for the loss she has suffered.
Even if the notice was valid you should have applied for a Court order for possession rather than changing the locks and this is probably an illegal eviction.
She probably does have a right to call a locksmith on the basis of the questionable notice and illegal eviction, or to sue you for the loss you have suffered I'm afraid.
Depending on your agreement with them your agents should have provided you with more information about legally evicting tenants.
Sorry it could not be better news.
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