Anyone know what the plant is in the attached photograph? Looks a bit like a primrose, but behaves quite differently with the flowers opening slowly and then going white after flowering. I got it with a batch of mixed plants and have lost the labels. It spreads quickly - I started with one last year and now have twelve!

Yes, it's Primula...maybe P. denticulata?
Drumstick Primrose (Primula denticulata)

Welcome to the forums.

Or maybe Primula capitata.???
https://en.wikipedia.org/wiki/...

1 - 3 Purple flowers are are Primula capitata.
4. White flower is Primula denticulata.

P. capitata DOES look like a better match.

Thanks for all your replies. It is definitely Primula capitata. I have found it on the RHS database as well. A really unusual flower form. It is spreading very well in my garden and I have been using it in several places - all from one plant!
Thanks again
John