Dryas octopetala (Mountain Avens). It is more of a creeping sub-shrub and tends to grow in limestone mountain regions across the northern hemisphere, including Scotland - the emblem of the SRGC!
I am slightly surprised to see it in East Yorkshire but, hey, why not. It will probably love your chalky soils.

I picked it up for 50p at a local HPS talk and its in a novelty cactus planter at the moment.
I had a P in mind for its name so I am glad I now have an accurate name for it. Thanks Bulba.
Does it propagate easily or more to the point if I put it in the rockery will it run? Yes I am on chalk so the alkaline conditions will suit it well.

I have this growing over paving beside my pond for at least 16 years. It not only has pretty white and yellow flowers starting in the spring it also has lovely seed heads. I never do any thing to mine just leave it alone except some springs I do trim the woody edges to contain it but that is all I ever do. You will love it in the right place as it makes a thick ground cover carpet. I have several photos of it in my file.

Firstly, Sbg, you got a bargain! If it is happy in the rockery it will not run, as such, but form a wide mat. From seed I find I get a low germination rate but it does well from cuttings or Irishman's cuttings. And yes, Drc, the seed heads are as good as the flowers.

I didnt mean to imply it runs mine is just a thick carpet of which I just tidy the edges occassionally

oh this does sound like a bargain then. I will have to think about where to put it in the rockery. Some things I have planted in the rockery are not really suitable. the Rock Rose in particular is too wide now. this would go in its place nicely. The fact that it is slow drc means it would be good.

I've heard of Irishmen's cuttings are these the self rooted bits or is it something else.

Yes, cuttings with small roots already formed.
Never got any seeds to germinate, but ordinary cuttings are easy.
There are other Dryas, worth looking out for. Dryas x suendermannii is one I used to have.

Thanks for the clarification owd :o)