Leaves look like the weed common vetch, which should have purple flowers soon.

Its one of the yellow forms of Polemonium. P paucifolium possibly.

Yep, completely missed the yellow flowers until I rotated the screen. Mr Brain wasn't up to looking at it sideways.

Thank you Seabg., that's what it is. It seeds itself around easily & I was going to give this one away(it's in a pot), but it rooted itself through the bottom & now is hiding the compost heap nicely, so it gets to stay!

Could I be cheeky and ask for some seed? I lost the ones I grew several years ago.