What is this plant?

No, I don't think it is Comfrey either. I've got two kinds and it bears a resemblance but you are right, this one is more upright. What are the roots like? Comfrey grows from deep running roots, and comes up again from tiny pieces left in the soil. They have been in flower here for some weeks, too. Sorry I can't help more.

To be honest it looks more like Verbascum thlaspi. This is a self seeding biennial with yellow flowers. Either that or it is the thing we know as Lamb's ears.

It's definitely not Lamb's Ears - that stays low and spreads horizontally. I'd go for verbascum

Definitely Verbascum common name Mullien which variety or species cannot be sure till it flowers. Healthy looking specimen suggests a flower spike potentially of 4 to 5 ft in height likely to be produced. Best of luck

Ours get absolutely decimated by mullein moth. In fact I only grow them for the moth rather than as garden plants!

Thanks all for answers & pointing me in right direction. I think it looks like Mullein (Verbascum, don't know which type) from further investigation..."The leaves (first season) at the base of the stem form a rosette of numerous, large, 6 to 15 inches long and up to 5 inches broad, but become smaller as they ascend the stem, on which they are arranged on alternate sides. They are whitish with a soft, dense mass of hairs on both sides, which make them feel very furry and thick. The root is a long taproot with a fibrous outer cover and fleshy inside. The flower-spike (second season) has been known to attain a height of 7 or 8 feet, covered with densely crowded, sulphur-yellow, flowers about an inch across with five rounded petals. Blooming during July and August." - Article by Deb Jackson & Karen Bergeron, found on internet.