Grey discs stuck on dead sweet chestnut leaves, about 5 - 8 mm across, no sign of a stem.
No interactions present.
In pictures 2,5 I can see what might be a yellow segmented larva inside. Did you open some to have a look?
Hahaha... no, I didn't think of that!
Lat/Lng: 52.062369, 1.155181
OS grid ref: TM163451