Buttercup ?

Geum, geranium, potentilla? Probably geranium but a flower will help.

my first thought was geranium.

My vote's for Perennial Geranium.

Must admit i was thinking about this at work today and looking at my geranium splish splash coming up....they did look similar !!!

Geraniums do self set, I had one called Claridge Druce
it is a thug, it pops up everywhere, in really large clumps, to be avoided in my book.

there are many geranium species so the fact that you have different leaves is normal. Bit like tigers and lions really.

I did think buttercup at first Andy , I think it grew last year in the same spot and I dug it up as soon as I saw the leaves. It has formed a lovely mound so I wondered if it was something else.
I think I will have to wait to see if it flowers Bulbaholic as you said, to find out.
I will check out the ones you suggested Lizzie and Andy.
Looks like time will tell SGB and Sheilabub.
Thanks for your replies.

Just an update, the first flower has arrived and it is pink and definately a geranium. Thanks for all your help.