I was flying to Berlin last year when there was a similar competition "in the area". Bremen Info had warned us about the glider competition "at this airfield" and the airfield was also in the NOTAMs, which I had read. My route took me about 50 kilometers north of said field so I thought I was in the clear.
Surprise, surprise, when I saw a stream of 50+ gliders crossing my track, same altitude. Fortunately so many that they were easily spotted from far away. Climbed above the bottom of the few Cu-clouds that were about and I was in the clear.
But the thing that struck me most was that Langen Info didn't know about the task set for the day and the route that that task would mean. They only knew about the starting location. Or, at least, they only gave us the starting location as part of the FIS.
And of course none of the gliders carried transponders, so Langen had no idea on where the pack was, or how high. If only 10% of the gliders would have a transponder, in contests like this, then this makes the pack very visible on radar and PCAS/TCAS.
So I hope that the contest organization will inform the FIS (sorry, Basic) providers in the area (not just the LARS providers as suggested in the OP, but London Info as well) of the actual tasks set, the actual starting time and the estimated maximum altitude of the gliders, so that they can inform others.
And yes, transponders and/or Flarm in gliders is a separate discussion which is already running in a separate thread.