At last week's Charter Christmas fair, there was a conman using fake £20 (and trying to use fake £50s). We rumbled him but failed to stop him getting away (and also failed to get a photo, which we're kicking ourselves for now!) The fake notes were hard to spot (we only realised after he bought 2 hot dogs in quick succession with 2 x £20 notes); they have holograms and strips but the colouring of the Queen's head is a bit different; you can only really tell in bright light and when you've a real note to compare it to.
Apparently one way round this is to have a 'bank' at the Fair where people can change their £20s for £10 notes and the person doing the changing knows how to spot a fake. That's what we'll be doing next year!