Thanks lph. Just turned the bike over to check. The cable routing is fine but I've just noticed that maybe the cable is not in as good condition that I thought as there is some rust down there.

I can feel the cable tension changing when shifting up but there doesn't appear to be movement as I shift down and the derailleur is sticking on the downward shift too. In fact I'm noticing that the shifter is also sticking. Instead of springing back up automatically, I'm having to nudge the thumbshifter then everything moves. I have lubed the derailleur but possibly not enough on the pivot points. Will try what you suggested.

Thanks again.