I think (and pray) this is doing the trick! I attach a couple more pics - 1 of the baby that was really suffering (seems to be in a better state of health today) and another of one which was showing symptoms on the middle fan leaves (this is getting worse but I hope I've stopped the rot, so to speak).
A question has now occurred to me. Should I remove the damaged fan leaves? I'm guessing I should (cleanly of course) but does it do any harm leaving them there to fall off naturally?
Thanks for a reply.