Thanks for both of the above comments,
The main window in question has an overall frame size of 140cm wide x 110 high, within this 3 actual double glazed sealed glass "panes"
1st glass pane, 30cm wide x 100cm high, this is an opening window.
2nd glass pane is not opening, but is 40cm x 100cm
3rd glass pane, same as 1st, an opening window ,30cm x 100cm
There is NO cavity whatsoever, the house was original 9inch red brick, with a pebbledash finish (that has been painted god knows how many times over the years) then they put this 7.5cm thick, compressed fibreglass type insulation on, called "vensil resil" (or something like that)!, it comes in 180cm by 90cm by 7.5cm slabs, and is drilled straight onto the original painted pebbledash, leaving no gaps, and then it is plastered with something called "alumask" and then while still "sticky", some guys obliterated the house by throwing tiny marble/granite/stone chips, at the wall (making as much mess as possible ha)
My beady eyes watched the whole process, and I didnt notice any gaps or cavities above the windows at all! they just stuck some false looking buff bricks at the top of the window to make it look nice! (cough cough)
But Im pleased its not a big fire safety issue, its just made the problem of moving my large 12ft leather corner settee an impossible task!