Is it still in the pot, or did you plant it in the ground? have you been feeding it, and if so, what with?

Isn't 'Nelly Moser' also an early flowering type, Bamboo?

Yes, Tug - I'm also curious about exactly when it was moved and whether it was pruned.

I moved it out of the pot and pruned it about October last year, I've fed it with slow release fertilizer and then with rose feed about February/March. I've now gone and bought blood fish and bone!!.. is it worth keeping?

Ah, now there's the explanation - you pruned it in October. Nelly Moser is one of those clematis which actually needs no pruning, produces large flowers roundabout May through to June, then produces a second crop of smaller flowers in about August/September. Your pruning in October means you cut off the wood which would have flowered in May - you should, though, get the second, later flowering. Don't prune it if you want the large flowers next year.