Well as I have maintained the ast ends and an spt begins which is in law a new tenancy however they have deferred in the past. That means if they are taking hard line then it is merely a further notice when the spt begins and that it until there is new letting. Thats why it is better, unless the lease trips you up, to grant a longer AST with a break clause.
Based on the information posted, I offer my thoughts.Any action you then take is your liability. While commending individual effort, there is no substitute for a thorough review of documents and facts by paid for professional advisers.