Hi Jon,
some "unseen" RAF camera gun footage survives at the IWM in London.
http://www.iwmcollections.org.uk/qryFilm.asp
They have an online catalogue of their film holdings but a cursory search using the terms "gun camera" and "camera gun" shows, ironically, that most of the footage appears to be from 1940-44. In addition, the weekly war news segments, shown at theatres, may have included such a clip.
However, I would think that the only way to find out for sure would be to visit and view the footage...
Cheers
Rod