Plum tree leaf problems

For the last couple of years my plum tree leaves have been quite off - to say the least.


Can anyone tell me why they are distorting like this,..



...and how do I treat it / hep the trees?

I have tried picking off the affected leaves (l-o-n-g job) and burning them, and thought that last year I had sorted out the problem, but obviously not.

It didn't seem to affect the harvest last summer