Small dark brown larvae/pupae/nymph of some kind. About one inch long. All wriggling a lot. Bodies with lots of segments, 6 legs near to the head end. .....
Seen in a damp drain area under a road in a large urban woodland park.
On this walk I spotted all these wriggly things on my way back to the vehicle, there was quite a mass of them, but even as I stopped to peer at their weirdness they seemed to all wonder off and disappear. .. not all , but by the time I took the photo, nearly half of them had gone. .. who knows where. weird! Thanks Martin for the identification!