NO4998 : Come for a Glider Flight Dinnet Airfield
Anne Burgess has corrected my assumption that a competition was in progress thus:
The competition was (as usual) in the first week of September. These are indeed visiting gliders, probably from England, but they're not at Deeside for the competition. Deeside is a very popular place to come and fly in the autumn because the strong winds in September and October create mountain lee wave, which is the Holy Grail of rising air for glider pilots, and doesn't occur in the south of England where there are no mountains.