Good points Gertrude.
Yes, I had been observing the a/c so was prepared for evasive action.
When flying in a gaggle with other gliders lookout is essential. If the other gliders are on the same level it is not easy just to turn to exit the thermal (we tend to fly in very close proximity). This has caused mid-air collisions in the past and will continue to do so in the future. Thus an aircraft that is on a conflicting course will present an increasing hazard as the number of gliders in a thermal increases.
I would not advise calling a club (telephone) to find the number of gliders that are operating. It may be of use to know that a club is flying, however that does not account for any visiting gliders which are allways about on good days.
When I am flying powered I try to keep well away from gliding sites. If passing very close I will call on the gliding frequency and announce myself just to keep everybody happy.
My main concern is for all pilots to maintain a good lookout and not rely on others to get out of their way.