Unfortunatly my computer will only show half of the picture. From the look of the hind legs the flattened segment suggests a member of the Portunidae or swimming crabs. They have long delicate chelae and from experience can give a pinful nip.
I hope this helps.
Thats better. There are two swimming crabs that come to mind the sandy (Liocarcinus depurator) and the marbled. (Macropipus marmorata) Sadly I dont have the books I used when I was in Fisheries so I can't be positive. If I remember correctly it is a case of counting and measuring the indentations on the front of the carapace.
Hope that is a bit nearer. Any book on shore creatures should have both species in.
Lovely stuff Tony, that narrows it down. I might pop in to the Centre for UK Biodiversity at the Natural History Museum and have a look at their reference books at the weekend as I have a few idents from a couple of holidays to complete. Not knowing a great deal, the juveniles and instars of insect larvae usually have me stumped!
Thanks for the help,