Well its only a weed if you don't want it where its growing ... not sure what it is,I always find it easier to ID something if I can see the whole plant to judge its growth habit and size, but maybe someone else will recognise it immediately.

Thanks,it was growing in a cluster,one a wall,I think it looks a bit like honeysuckle bit that is usually a climber,but the house is empty and the bit of garden it has,is well unattended.

Is it a type of corydalis, I think I've seen it about in crevices etc

Yes, a corydalis and very pretty. I couldn't consider it a weed though I doubt if anyone plants it intentionally.

I grew it in Stafford where it was very happy in the light soil, where the blue ones wouldn't grow. Sorry Bulba, I did grow it on purpose...

it is Corydalis lutea and can be a weed as it seeds very freely. I have small areas where I encourage it and the the rest get weeded out.

Thank youfor all your replies.