It seems possible from the four spots on the rear end of this little chap that the species represented here is Limnoria quadripunctata Holthuis, 1949; but in the absence of any online gribble guide it’s hard to be sure. Any gribble experts want to speculate?
[Update: see comments below for elucidation]
UPDATE: The release of the gribble armada After our brave little TV stars had managed to survive over 24 hours away from seawater they were starting to look a bit sluggish when asked to perform yet again under the lights this morning at work. The Ranger decided they needed to be given a fighting chance to go off and nibble some toff’s yacht in Cowes. So down to the river Medina went the gribbles, wrapped in a magazine, and they were sent off in a little flotilla of wooden fragments.