Ridgewell gliding site (UK) is just outside the controlled airspace for Stansted Airport. This is a choke point for traffic transiting from South East UK to the midlands or points west (some of which goes on to overhead Martin G’s site).

A recent airprox report included the following:
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Summary

An Airprox occurred in Class G airspace overhead Ridgewell gliding site between an ASW20 glider and an A210; both aircraft were flying VFR.

PART B: SUMMARY OF THE BOARD’S DISCUSSIONS

The gliding and GA members opined that the A210 pilot appeared to have either planned his flight to pass over the gliding site (through the circuit and well below the winch height), or that he had not planned well enough and was unaware of the gliding site. Both options were seen as equally dangerous and a fatal crash would almost certainly have been the result of his aircraft hitting the winch cable or a launching glider. [snip]

The Board opined that the annotation of Glider RTF on VFR charts and within the AIP ENR5.5, would greatly assist in this respect by providing a readily available contact frequency for those GA pilots who planned to fly close to glider sites, and who wished to determine their levels of gliding activity.

[Chris N note – many small UK gliding clubs do not have site base radios, or if they do, they are often not manned. There is no legal requirement for such radios. Nor is there a legal requirement for gliders or GA aircraft to carry or operate radios when flying in class G in the UK, though increasing numbers do. ]

[snip] . . . It was also noted that many of the larger gliding sites can have winching up to 3000ft agl.

Ridgewell is marked as a gliding site on the 1:250,000 and 1:500,000 series charts and so it is reasonable to expect the A210 pilot to have known of its presence; he would have been prudent to have given the site a wider berth, especially as he was flying at around normal circuit height. In addition, the onus was on the A210 pilot to give way to the glider, although the lack of any enhanced conspicuity features on both aircraft would have made visual acquisition by both pilots more difficult. Although there was likely to have been around 200ft between the aircraft, the A210 flew directly over the gliding site and over the ASW20, whose pilot had few options to avoid the other aircraft. The A210 pilot did not see the glider; the glider pilot saw the A210 and assessed that his flight path would keep him clear, albeit with safety margins much reduced below the normal. The Board concurred and assessed the Risk as B.

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I have seen many similar airprox reports over the years. There are typically a few every year (about 5 in 2009, the last year for which I found an analysis I did for the BGA). Interestingly, most of the infringing powered aircraft in recent years had transponders on and working. If gliding clubs invested in Zaon MRX or similar transponder detectors at the launch point or the winch, they would almost always give more warning of incoming infringers before the lookout person at the launch point could see them coming. We have one at Ridgewell, which I have tested on the ground and seen nearby traffic indicated several miles away. Unfortunately, no other UK gliding club has copied this (AFAIK), in spite of frequent incidents such as Martin and I have mentioned; our own unit has seldom been deployed, though some of us are trying to change that, but Zaon are now closed, I understand, so another manufacturer would be needed before any new transponder detectors could be so deployed.

Chris N