Sea slater (Ligia oceanica)
Update 09/02/11 This is now far and away my most popular photo on Flickr. Being the top hit on google image search for "Sea Slater" probably helps.
Update - I now suspect these are the common sea slater (Ligia oceanica). One of the 37 UK species of woodlouse.
A species of Trilobite alive and well and living in the Thames? Somehow I doubt it, so can anyone offer up an ID of this animal.
It was about 2cm ling from tip of antenna to tail. They moved quite slowly unless disturbed then they could put on a spurt of speed. They seemed quite happy in the water trapped between stones.