I'm not familiar with the Atom 80, but so many people reporting the same issues might point to carb icing (restricted venturi caused by the formation of ice when moisture freezes on cold surfaces as the pressure drops, or something like that!). Larger aircraft have carb heating, and carb icing is checked before take-off. The annoying part is that the ice melts quickly once the engine stops, so it can be very hard to diagnose.
https://www.boldmethod.com/learn-to-fly/aircraft-systems/dont-let-carb-ice-happen-to-you/
It was cold and moist in Salisbury, I assume there were similar conditions at Membury? Just a thought.