The bees love them, especially the red bottomed bees. I've just sat on the doorstep and watched them collecting the pollen. Some of them had so much on their little legs, they could hardly fly!
Definitely a member of the mustard family (Brassicaceae) which include, rape and mustard.
The bright greenness of the leaves and stems are steering me towards Black Mustard, after sifting through countless images, but I wouldn't rule out a hybrid.
What ever it seems like its a big hit with the bees!!
I have some of these in my wildflower patch. I've just had a look at the leaves and what I assumed was one variety of these flowers is actually two different ones...the rape has longish smooth sided leaves, the mustard has more rounder serrated edge leaves. The flowers are very similar.
It appears I have both.