having served a section 21 all you can do is to wait till it expires then apply to the court for possesssion - what dates are on her tenancy agreement and what dates did you put on the S21 ? This is a tenant who knows her L&T law and she will not allow you entry - even for a gas inspection.
Write her a letter re the missed appointment - - and make another appointment - she wont let you in again - then write another letter asking where was she ? and make another apointment - at least if it ever got to court you could then prove she was obstructing you and that you were doing your best to comply with the law.
Clottie The Positive “Windswept and interesting”
The Somerset-Lancashire lady
Aviatrix extraordinaire !