Back pain

Keep the big lock at work (or home) and then carry a mini-D for your short stops in between.

Unfortunately it's the stops in between where I need the size - the one-time cyclist that is my boss lets me keep it inside at work :)

how tight is it round your waist?

er, quite? I'm pretty slim and it sits around my hips - it's tight enough that it doesn't fall of when I'm standing but not so tight that I feel restricted when sitting.

it's possibly your position on the bike, maybe from pushing your hips forward and arching your lower back instead of pivoting forward at the hips and keeping your back straight.

I'll check this out next time I'm going by some mirrored buildings, but I think my riding position is pretty good.

Sounds like Sciatica. Not nice. Maybe the chain (cold metal on your spine) aggravated an unknown problem.

Yeah - I've had similar problems in the past (with my piriformis, if you're medically minded) so I figured this could easily be doing a similar thing. The problem is that the pain is always delayed by a few days after whatever is causing it, so the cause is always quite hard to pin down. Just wanted to see if this was a known problem.