Greetings, welcome to the Forums.
So it's in the U.K. too! Some of my
Yucca get this, and I've even seen the culprit, ...once! It's the larvae of a very small moth.
Read about the full saga here:
http://www.agaveville.org/viewtopic.php?t=400
If I am correct, Post # 18 reveals the answer.
So what is the solution? I recommend
Bacillus thurningiensis (
B.t). Pry apart as many of those crystallized leaves as you can and try to get the
B.t. solution deep into the leaf crevices. Good luck.