Godwin I did a photo search of the Imperial War Museums On Line Collections
www.iwmcollections.org.uk and found photograph number CH 18567 taken in December 1943 at the RAF station Leuchars which may have been inside a No.4 Dome Trainer. Does the sound and film projector in the foreground resemble any illustrations in your user manual for the No. 6 trainer? The gun certainly does not look like a 40 mm Bofors.
Brian