This grows in all the cracks in the concrete, and from the dry-stone walls as well. It looks like a hawkweed, but the leaves are variegated, and broader and more rounded. Multiple stems with groups of yellow flowers and dandelion-like seed heads. In flower from July onwards. I can't seem to find it anywhere in books or online, so any information would be appreciated.
It's possible that I brought it with me when I moved to Sheffield from Hertfordshire 25 years ago.
It's all over the garden and easy to get rid of by hand, but I quite like it so tend to leave it - hoverflies like it.