Many thanks for those answers, ChrisN - good practical advice, which I shall heed!
Do you have any idea how many non-PPL UK glider pilots have R/T licences, by the way? Just curious.
Referring to the original thread, it sounds as though you'd have been disinclined to call as part of the pack soaring that escarpment, unless there'd been obvious traffic working the zone - or perhaps if you were approaching cloudbase? I don't know the area, but it strikes me that it may not have been poor task-setting that took them close to a busy zone - a working escarpment could well attract you a few miles off an area of sink on track, and a few miles can be quite a lot in English airspace...
Tim