A Bank Holiday visit to the woods, under the overcast sky. It wasn't raining (yet), but there was a soft and steady pattering sound emanating from the tree canopy overhead. Venturing closer, this was revealed to be a regular green rain of many fragments of knopper galls, tumbling down from above. Knopper galls are misshapen acorns, stimulated to modified growth by a tiny gall wasp. I'm guessing these were being nibbled open for the wasp grubs inside (by squirrels? I couldn't hear any nuthatch hammering noises).
For more info about knopper galls, see https://en.wikipedia.org/wiki/Andricus_quercuscalicis
At home, a hitchiker emerged from my backpack: a strange invertebrate, flattened like a flea and with flea-like clicking, jerking jumps, but, at 1cm ish long, far too big and with far too many legs; a sort of flea-woodlouse.
No hints in any of the books. Some quick Googling of 'terrestrial amphipod' suggested it was a 'Woodhopper' or 'Lawn shrimp' Arcitalitrus dorrieni, an introduced species from Australia, now widespread in the south west among leaf litter and damp detritus in woods and gardens: