Thanks for the advice bonbon. I'll give that a go today and post an update soon.
As you may have worked out by now, I have recently bought a new digital camera, hence all the pics attached! I'm gonna attach the last two now, as I hope they may serve other growers in future. They're of another plant which hasn't got the curled leave problem but the middle fan leaves are starting to discolour. I'll give these the same treatment as the sick one as I'm sure they'll all be lacking the same thing!