Green Alkanet (Pentaglottis sempervirens). This plant grows all over London. Where one grows more will follow and many gardeners regard it as a weed though I don't.
 I was asked about it when I took part in an event yesterday in my neighbourhood entitled "Greening the Hill" (referring to the Telegraph Hill district in south London). It included a gardeners question time type session and I was one of the "experts" taking questions on the stage! I haven't done something like that before- it went fine if a little nerve wracking to find myself talking in front of an audience.
 What I said about Green Alkanet is that if it was rare people would pay money for it. The forget-me-not like flowers are small but show in profusion for months over the verdant foliage and it's a top bee plant. The trouble is it isn't rare.
 On the contrary it spreads vigorously with deep tap roots that make it hard to eradicate. If you pull one up and a bit of the root remains another plant will grow from it. The point I went on to make is that if you look at a list of UK wildflowers and a list of UK weeds it's basically the same list. So one person's weed is another person's wildflower.
 I saw this plant's invasive qualities used to stunning effect in Kew Gardens a year or two back. It had been allowed (and I think encouraged) to spread along a lengthy section of the perimeter wall with another plant that multiplies rapidly: Symphytum orientale, a comfrey with pure white flowers about the same size as the blue flowers of P. sempervirens. The foliage of both is rather similar too so the greenery provided the setting for an extraordinary haze of blue and white shimmering in the sun.