"If you fly at 3000ft+ then I am sure nearly all are Mode C/S equipped and transponding"
As a converse example, on Saturday 23rd August there were (NOTAM'd) gliding competitions at Dunstable, Lasham, Bicester, Pocklington, Husbands Bosworth and Aston Down. These alone will have accounted for around 200 gliders airborne between 12:00-16:00. Cloudbase in the task area (basically most of Southern England) was between 3000-6000'. Therefore I'm pretty sure there'll have been several hundred non-transponder equipped aircraft operating between 1000'-10,000'. Anybody out flying that might day might ponder on how many gliders they saw (and importantly, how many they didn't).