We've had a run of plastic £5 notes in circulation in scotland for about a year... it's not really a huge problem. I handle fairly large volume cash transactions on a daily basis and at speed - this has always been a slight issue with new notes. In my experience, paper £10 notes are the worst for sticking. The polymer ones actually slide apart more obviously during counting than new paper ones.
This is why the first thing I do when I get a stack of crisp new notes... is crush the **** out of them

. Then alternate them front-to-back and voila - no chance of two sticking together. It's also a strangely satisfying feeling. They're so fresh and perfect... sometimes you just want to destroy something beautiful...