a busybody neighbour perhaps...
Since I would expect the majority of people will know nothing about this legislation and its requirements, chances are they would in all good faith answer such questions assuming changing a window would not be included. Much will depend on how specific the solicitors are in phrasing the questions while conveyancing I guess.
Those who do know about such things are likely to keep schtum, or have a slight error in recollection of when the window was installed! ;-)