Adrian,
Thanks for the PM. Did a spot more research today - in the form of going to look again at the book, since these Air Britain works are right much more often than they're wrong, and I had to be certain that I'd read it correctly.
Discovered that if one opened the book a little wider to overcome the slightly over-tight binding, the magic letters 'A&GS' appeared at the edge of the page...
So the Grebe was with the Armament & Gunnery School at Eastchurch when it crashed.