I looked into this a few years back so any legal information might be out of date - check with the EA if in doubt.
An attended drop net is perfectly legal - I once took 19 out of a Kennet carrier in half an hour with a single drop net and a bit of rather high rainbow trout as bait.
I remain to be convinced about leaving them to get on with it - there aren't any stable populations in the UK on which to base a reliable hypothesis - increased predation of the little ones by the big ones encourages the little ones to move elsewhere and colonize new areas in just the same way as pressure of numbers does. However, a fully grown signal cray has no natural predator in the UK - they get to 30cms or a foot long. The juveniles and smaller crays are eaten by fish and are less able to evict water voles from their burrows as the large ones do, so there's an argument for keeping a signal crayfish population small instead of large just as there is for keeping them large instead of small.
Things to make sure of:
1)
Identification! - only take signal or turkish (narrow claw) crayfish. Whiteclaws are protected and the fines for 'interfering with them' are prodigious. The other species in the UK (Noble, Spiny cheeked and Red Swamp crayfish) you're unlikely to come across and also bear the strongest resemblance to the natives (Red Swamp excepted - they look like nothing else on Earth) by not taking them or catching crays where they're in evidence you're less likely to do something silly. Don't fish for crayfish anywhere where white claws are know to still exist - they're in serious jeopardy and need as little interference as possible.
2) If you're fishing several different locations, make sure you dry your equipment thoroughly (at least two days for a net in a warm place - no, the unheated garage in winter isn't warm enough) Aquatic diseases are normally killed by being thoroughly dried out - crayfish plague spores certainly are.
3) It's illegal to release non-native crayfish back into the wild after capture - if they're captured then they must be killed. It's also illegal to keep them, outside of the areas for which there is licence exemption, for more than 72 hours if they're being kept for personal consumption.
4) Unattended nets / traps are likely to be classed by the EA as a 'fixed engine' and you need a licence to use those in fresh water - again the fines are prodigious.
5) Whatever you do don't be tempted to 'seed' an area with signals or any other crays. The habitat destruction has to be seen to be believed and while you might get an occasional crayfish starter out of it, you'll lose more or less everything else in a water, right down to the reeds when they get hungry enough. The Serpentine in London is stuffed full of Turkish (if memory serves) crays - there used to be a couple of blokes with the licence to trap it and they'd take a couple of hundred weight a day out and it would just keep the numbers level. There's just about nothing else in there now though - not even weed.
Link to the EA:
http://www.environment-agency.gov.uk/homeandleisure/recreation/fishing/38045.aspx
Hope that helps someone,
