For me I have witnessed 3 mid-airs involving gliders - 1 at Lasham (non fatal), 1 over Cranwell North (2 syndicate partners killed in the glider - the Robin Aiglon landed Ok Southside) & 2 gliders at Colerne (2 killed - 1 a syndicate partner).
I always ensure my students keep a hell of a good lookout & others are wary of me joining their syndicate!