Please note some traps for sale on ebay DO NOT comply with British law, and please also note that you DO need a licence to use crayfish traps of ANY sort contrary to what some sellers are saying.
Trap sizes must comply with these sizes: Opening size no bigger than 200mm, Mesh size no bigger than 30mm, length no more than 600mm and width/height no more than 350mm. Failure to comply with these regulations can lead to large fines please CHECK WITH SELLER BEFORE BUYING
Licences are available free of charge from the environment agency along with all the information you will need concerning various bylaws etc, again failure to produce a valid licence will result in large fines.
There really is so much legislation to fall foul of when crayfish trapping so please read up thoroughly on the law and do get in touch with the environment agency BEFORE buying traps it may even be illegal to use them in your local river WITH a licence.