I've seen a couple in Brighton and Telford. They're quite rare and very difficult to spot, especially when they're in a crowd. Use binoculars to identify them from the givaway sign - black burn marks on the fingers, colloquially know as Kentucky fried fingers.
The best way to catch them is with bait. Get a normal bike and tie a few random electrical components to it - the more the better- then park it in a place where thousands of people walk past. Normal people will not notice what it is, but an ebike mechanic will figure out that something is wrong, so they'll approach and start inspecting it. That's the time to throw the net over them. It's best to put all the elctrical stuff low down so that they have to crouch to inspect, which gives you a better chance of getting the net over them before they try to escape.