More mental than physical fettling, but I worked out what the slight rattle on the back of Sid (the Singlespeed) is. Ever since I started using him again on Saturday (so for about 50 miles now), there has been a very slight rattle. It seems worse when I pedal hard, but is definitely still there when I freewheel.
After thinking it was the cable to the rear light (nope), I eventually twigged that it was the mudguard stays. When I put the Kaffenback together, I dropped one of the small bolts that locks the stays to the mudguard proper. I couldn't find it (obviously suffering from something akin to Fridge Suck), and "borrowed" one off of Sid until I found it. The nut doesn't seem to be one I've got in my spare bits pot, and I've yet to find the one I dropped.
I suspect putting a cable tie around the stay tonight will be enough to temporarily stop the rattle until I make a determined effort (again!) to find the missing nut.