Thanks for all your suggestions. I don't think it's Centaura Montana - the leaves are too broad. And I know what you mean about it being like a foxglove, muse, but again, much bigger than any foxglove I've ever seen! We have evening primroses in our garden, Ro60, and the leaves are very much smaller than this monster!
I think Fleur and Avalon have got it right - it's definitely looking like a Mullein, probably the Great Mullein.
CafeAuLait I just put it on Plant Snap on my phone and it came up with Verbascum Thapsus, which is the proper name for Great Mullein, so there we have it! (I forgot I had that app - I stopped using it because it so often gives random results!)