does anyone know if I need a license to catch these and where do I get it from
You need a license from the environment agency if you want to trap signal crayfish. If you ask via their contact page they will send you a load of forms to fill in. Alternately crayfish trapping advice packs are also available from the National Fisheries Laboratory who can be contacted on 01480 483968.
Their EA crayfish pages have a bit more info:
http://www.environment-agency.gov.uk/subjects/fish/246986/342184/1205879/?version=1&lang=_e
I catch them all the time, have done for years - never bothered with a license ;)
If your going to get caught 8) or if your going to leave the traps out over night (which i dont), or if your trying somewhere with native crayfish - then i'd get a licence. Where i go there are just 1000's if not 10000's of signals...
Reminds me of somthing i saw on TV last week. Hugh Fernly whats his face caught a load of signals on the kennet. He cooked only the tails of the big ones, and then seperatly cooked up the little ones (whole) and the rest of the big ones seperatly to make a "stock" - well unless you like your "stock" mostly consisting and tasting of Crayfish poo - i would'nt advise doing this..