Went out to fix my bike today. It has a flat, I've got the patch kit, I can DO THIS.
Removed the inner tube and was faffing about with the pump, trying to inflate it. Oh. I have been using the pump wrong!
But... if I have been using the pump wrong, is there actually a puncture? Best find out.
Nope. No puncture!
It was flat because I deflated it myself, months ago, by using the pump wrong.
Woohoo, problem solved, I am a genius!
@HerraBRE I once managed to cause a leak just next to the valve. I suspect that I inserted the valve not perpendicularly enough and the force from the nut that forced it perpendicular was causing it to rub against some part of the wheel there.
@robryk I was gentle. I think it's all good!