Oh I'm so glad we don't have much of a a cray problem up here! My son who lives in Milton Keynes has been driven to distraction with them. Virtually no bait is safe and he's watched them in clear water appear from the muddy bottom to take his bait.
The only place I've come across them in this area were two ponds I used to fish at Trowey near Sheffield. The water was full of them, released I believe from a crayfish farm which used to be upstream. A good perch water and lobworms were a good bait - until the crays found them. Very slow dips of the float were a tell tale sign that they'd arrived.
Out of the water I've found them quite aggressive, waving their claws at you as they retreated backwards into the water. They are one of natures survivors and from what I hear, near impossible to get rid of. A bit like an aquatic cockroach (ish).