Thanks for the reply. Thanks also for that chart. That's brilliant and I hadn't seen it before.

The white spots are water marks from where the plant gets splashed when the patio gets washed down. I've just wiped them off with a cloth.

Sorry about the pic size. I wasn't sure how big to post so reduced it. Here's a full-sized one. I certainly notice the tracks on the leaves so it looks as though *something* is having a little burrow around in there. The problem started as those brown spots of the RH-most fan leaf and spread to 'burning' patches and on the leaf edges.

Cheers