I'm sure someone will know, Some poppies look like ferns when there growing oriental poppy etc:

Thanks Vic there is an ornamental poppy in the garden, it's leaves are similar but covered in hairs

I think it looks like Achillea millefolium (yarrow).

Probably an Achillea filipendulina, to my eyes. A. millefolium has shorter, "bushier" looking leaves with narrower segments.

I have a plant that looks very similar & it was identified as a fern leaved Corydalis. It gets little yellow flowers. However the others might be correct with their Achillea, it's difficult to tell how big & sturdy it is. I have a photo on my Questions page you could look at to compare.

I tend to agree with Feverfew. The Yarrow has a much 'bushier' close leaf. I think this is Corydalis cheilanthifolia which as said has yellow flowers and tends to seed quite profusely, or it might have come courtesy of the birds.

Could it be a tansy, do the leaves have a scent on them when you rub them?

Thank you for all your answers, I've googled them all and still can't decide what it is. Think I'll have to wait and hopefully it'll flower.