Just how does flying a cadet require such a huge MAUW? The BGA data sheet for a G103 gives the maximum load for each seat as 110kg including parachute, which is over 17 stone. And if flown at that MAUW doesn't it interfere with the CoG position? Or is it that the seat limits are unaltered and this allows for lots of heavy repairs? But if a glider is repaired to that extent, again I would worry about what has happened to it's CoG. Too far forward and it's nasty to fly, too far back and it's not only nasty to fly but dangerous.
Can most certainly believe the repaired gliders are heavier, one way of triaging before checking them would be weighing. Do the lightest ones first, reweigh the ones from the coast after a while in a dry storage area.