Looks very much like Rupicapnos, which is a weed. aka Fumitory.

Thanks. It is very pretty and she doesn't mind it being a weed as it gives a lovely backdrop to the rest of the flowers in the pot.

It is a weed but as you say a pretty one. If it came from somewhere exotic it would probably be extolled as a plant with 'beautiful ferny foliage and delightful purple tinged flowers', but as it's a native, it won't be! Lots of our 'weeds' are really pretty and delicate, if somewhat understated, but worth hanging on to as they are losing out to the mono-culture of modern farming and all that weed killer and fertilizer. Sad.

How right you are. I'm trying to discourage too much use of slug pellets for the sake of birds, hedgehogs and toads.

I agree that it's Fumitory, but probably not Rupicapnos; more likely Fumaria officinalis, which is as Honeysuckle says a British wild flower, probably an ancient introduction. I like it in my garden!

I told my daughter of all your replies and she's going to keep it because it's so pretty. Thanks all.

Warn her then that if its anything like here she will never be rid of it!I was thrilled when I first saw it but it gets heaved out now.

She's a novice, very enthusiastic gardener so anything that grows is a novelty bless her.