Well it might be Lynneg. but you haven't put up a photo so it is rather difficult t answer your question...

Oops

No, its not delphinium - looks similar to Anemone huphensis or herbaceous Japanese Anemone, but I'm not sure its that.

Definitely not a delphinium but no idea what it actually is!

Thank you all. Now that you've mentioned anemone Bamboo I seem to vaguely remember sticking a bit in which had broken off another plant

It looks like Japanese anemone to me too. It likes the right hand side of my garden and has been spreading itself into everything. I spend every summer trying to keep it in check but the roots are so deep they just keep coming back. Funny thing is, my sister has a nice neat clump which doesn't move at all.

If I recall it was only a broken bit and it's looking like a triffid! Will have to keep an eye on it

I agree with Thorney, it looks like the horrid pink Japanese Anemone which I spend half my gardening time trying to get rid of! You say your sister has a well behaved clump - is it the white one, which I think is better behaved?
As for the answer to Lynneg's question, keep an eye on it, unless you want a massive swathe of ground cover with pretty pink flowers, 2-3 feet tall.

I've been trying to work out why anemone spreads so much in some gardens and not in others - I planted the pink one years ago in a garden and it hasn't taken over at all, its still a neat clump, yet in another garden, its trying to take over. The difference is, the first garden has heavy soil, and that garden is north facing, so it can get waterlogged in winter. The other garden has lighter soil, gets some winter sun and doesn't get waterlogged, so I conclude it doesn't like heavy, wet soils in winter, and that's what keeps it in check. I can't think of another explanation...

I wish that was true Bamboo! We are on really heavy clay, some advantages - roses love it and the grass never dries out, but apart from that I try to make it better every year. The Anemone we have lives in this and thrives, gets mown down with the lawn mower and still comes through!
Our Bay tree suffocated (horrible thought) because the roots were wet and the leaves were frozen so it couldn't transpire or drain. It took two years to shoot out again from the base, after I had decided it was dead of course and planted a Wisteria to grow up the trunk!

Hmm, well, that scotches my theory then!