The whole set up seemed quite secure, though, so I decided to cycle instead. I was cautious at first, not wanting to take too many risks, but I do know the route to Wanstead quite well and the traffic on a Sunday afternoon is not exactly heavy. So off I went, like some sort of low-budget sequel to a certain film, and sure enough we got there all right. The journey back was in the dark but I have lots of lights (some would say I'm excessive in this, I prefer to think of it as "highly visible", which is surely a good thing!), and I was feeling confident enough to take a detour to a friend's house for a visit.
I didn't take any pictures, so you'll just have to imagine a great rectangular box with a red light on it sticking out off the back of the pannier rack. I'd definitely do the same again for short journeys -- but I'm still not happy enough with the case to try this method for getting to and from Quire.