There are a number of plants that this could be, and it's not possible to identify from this photo. However, my best guess would be Catsear, Hypochaeris radicata.

It's a yellow hawkweed.

Hieracum or yellow hawkweed yes - I like the orange version, but despite getting some from its natural habitat, it simply wouldn't grow in my garden...

The orange hawkweed is an abolute weed in our village, Bamboo, even though it is pretty.

It was down in Devon too, Bulba - that's where I tried transferring it from. But no luck - I've never seen in it growing in and around London for sure.

Not convinced about Hawkweed - they normally have very branched stems. Can't see the leaves but this looks more like rough hawkbit to me. Its a nightmare group of look alike plants to sort out. Strange plant to have in a border though.

In plant taxonomy class, we called them all D.Y.C.'s. The last 2 letters stand for yellow composite, and I'll let the imagination work on the "D"! :)

Nightmare, invasive or absolute weed, whatever you call it, the insects love it!
Reading a butterfly book yesterday it said some of them had larvae which fed on docks - now the question is when was the last time you saw docks growing wild in fields? How many of us let docks grow in the garden? So... where have the butterflies gone, sprayed to death with modern weedkillers on the fields, that's where!

Honeysuckle, the only nightmare is how difficult it is to distinguish between the different genera and species among the Composites. All of us have our least favorite weeds, but your point is well taken: unless they are an introduced species, they all have their place in the environment, with other species dependent on them.

Bamboo, be grateful you couldn't grow the orange hawkweed - its an absolute magnet for blackfly.

Thankyou all for the replies. I believe Landgirl100 nailed it with Cat's ear/Hypochaeris radicata.
Found an great American site, after I'd put the question out there, which had photos off all it's life stages and perhaps because I was able to zoom in on my own photo of it, the one you've all seen, it matched up without doubt.

Rough Hawkbit was a new one on me after much online trawling, but after viewing a close up of it and it's hairy bud stage, Cat's ear was super confirmed.

So thanks again everyone.