I think it is a red/orange hawkweed and if so an invasive weed. Photo of leaves would confirm.

It is indeed Orange Hawkweed.

We had this in our previous garden and it was a royal pain to remove - very pretty to look at though and butterflies and other insects love it.

Yes, I have it too. I try and keep it to a minimum. It is quite prolific at reproducing itself though, what with the runners and the seeds! I agree, quite pretty too. I like one of it's other common names, Fox and Cubs.

Grim the Collier, Hieracium aurantiacum, and another name which escapes me. As invasive as dandelions.

Pilosella is their new genus name. The botanists have been at it again and separated the rhizomatous members from the rest of Hieracium.

This is now Pilosella aurantiacum.

That is the name I was trying to remember. My brain is definitely beginning to fail!

Thanks to you all.