Another 3 hours or so of work done on this. I put a layer of Phthalo Blue and over that some more Sky Blue, in places on the background just to give it a bit more ooomph! My lighter blue pencils are now reduced to little stumps - oh the perils of sanded paper!
I have basically strengthened all the colours on the petals and tried to get more tonal contrast, more by lightening the light areas than putting down darker greys - if that makes any sense. It's quite satisfying being able to put lights over dark colours, as you can with this paper, and I have used a Chinese White Derwent Drawing Pencil as the final layer for the highlights.
I think it's probably about finished now. I'll put it away for a few days and then look at it again. No doubt something will shout at me to be tweaked, I'm never satisfied.