I've just identified this as Impatiens glandulifera Royle (Indian Balsam) using an online wildflower identification questionnaire. I thought the leaves looked a bit like overgrown Bizzy Lizzies.

The plant's common names are Policeman's Helmet and Jumping Jack, although in the USA it's rather grandly called Ornamental Jewelweed. Originally from the Himalayas it is now common in the wetter areas of England and spreading... but at least it's not a perennial!

The weather's gorgeous this morning, although the clouds are beginning to build up now and I expect the sun will disappear soon. That's ok, I don't mind overcast at the moment... but I wonder if I'll still be saying that next year? In fact I've just noticed from my year-ago-blip that I probably won't be!