Do gliders religiously speak with any FIS/RIS providers? Thats surely the safest way to alert aircraft of a cluster of gliders in a particular vacinity.
Instead all we hear is,
"G-**** keep a good lookout, multiple pop-up traffic in your vicinity, believed to be gliders, no height information".
Translated as, "G-**** good luck, loads of gliders in your area, all over the shop and I know nothing about them"