Oxford Visionary - 7 quid for a pack of three The greatest invention to hit biking since the Doctor Marten boot. Simply put, it's a visor-shaped bit of tinted plastic with sticky bits at either end. You stick it behind your visor (I need to trim it slightly with a Stanley knife to allow the visor to close properly) and, hey presto, one black visor.

But, and this is the clever part, as it is not a permanent addition to your visor, it doesn't invalidate the BSI kite-mark, meaning that the visor stays legal. If it does get dark, you can whip it out and roll it up (quiet in the back), and stick it in a pocket so you can re-use it (the sticky stuff gets too cruddy to re-use after four or five goes of this.) 

And as a happy side effect, the fact that it leaves a tiny gap behind the visor means that it works like the pinlock system, reducing visor misting significantly. Go get them.