The procedure is the BHPA investigate all accidents, however, most incidents are not serious, relatively speaking so they stay within the gliding fraternity. Any death or serious injury is notified to the AAIB & they get involved at their discretion, as do the police.
If Ashbourne's information is correct, it appears to be a case of lockout under tow. static towing obviously produces a high angle of attack and leaves the towed aircraft much close to the stalling point than it would otherwise be in normal flight - power towing less so. If an aircraft deviates significantly from the tow direction, it becomes dangerous as safe turn inputs are reduced due to the angle of attack and either lockout or a spin can occur depending on the circumstances of the tow. The solution is to cut the tow line, lower the nose & re establish a safe course, in that order.