If the ceiling void is well-insulated, why is it cold beneath the insulation?
Uh long story, there was a mistake with the size of the windows and the height of the ceiling. Essentially the ceiling had to be lowered slightly by 50mm so that it can line up with the rest of the open plan ground floor. The windows however have already been sized and fitted. Essentially the 30mm void between the plasterboard and the vapour barrier lines up with the the trickle vents of the windows essentially allowing a breeze to blow through the void.
The temperature isn’t very cold but you could feel a slight temperature variation.