Folds up its back legs and jumps. Lots of them are found on Great willowherb leaves.
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I used to live by Morden Hall Park, and remember these well. I even reared a few, just to check that I could. :o)
There are a couple of other candidates for this, but I think lythri is the most common by some margin... so yes, I'll agree with 'probably this'.
Lat/Lng: 51.405, -0.1889
OS grid ref: TQ260688