Cheers for the info Guys.
I got them out of a land-drain (about 4 metres wide) under a motorway bridge. Most of the time the water in that drain is clear but I think there's a small sewage plant on it a few miles up stream. So I think I'll play it safe and just use them as bait for fishing. Its a pity they can be so dangerous because they look to have a lot of meat in them, 4.5 - 6 cm in width, 7.5 - 10 cm in length.
I looked in my Field Guide and they are the more common variety.