Hola to you back, Graeme,
I've been on holiday, so only just seen your post. Sorry for the delay in responding.
My understanding is that the use of the word enclosed with the word stairwell is only to differentiate it from those sort of stairs that may be found in otherwise open areas. Say from the ground floor to a balacony in, say, a big library reading room or similar.
The idea of making the detectors at various levels in these enclosed staircase areas part of the same fire zone is surely to avoid the confusion of the smoke from a fire on a lower level reaching a detector on a higher level whilst by-passing one or more detectors on intermediate levels. If all these detectors were actaully on different zones relating to the various floor levels and as part of those zones that were extending beyond a door between the staircase and, for example, a corridor, it could be very confusing working out on which level the seat of the fire existed from viewing the operated zone indicator.
In my opinion if there is no door between the stairwell and the corridor then I believe all the detectors in that corridor should also be on the same zone as the stairwell detectors. This scenario is hardly likely because there is normally always a fire door pretty close to the actual stairwell and which would form the fire zone boundary of the stairwell zone.
The question you raised before was what if there was a room that led straight onto that stairwell at one level. You wondered if that room should be part of the stairwell zone or the floor zone. I think it could be part of the stairwell zone, although it might be less confusing if it had it's own zone entirely (if possible). But this sort of room probably shouldn't be part of the zone that starts the other side of the door to the stairwell at that level, because then you would have to travel through the stairwell zone to reach two separated parts of the same zone when searching for a fire condition and it may confuse people trying to establish where the boundaries of zones are.