Walking down Charing Cross Road a while later, I realised the ring wasn't on my finger. Somewhere, somehow it had fallen off (probably while I was fiddling with the strap on my heavier than normal bag).
Urk. Not good. Time for a little panic, and then a concerted search.
I retraced my steps, all the while knowing that someone would have picked it up and walked off with it. After all this is London we're talking about...
Luckily, for once, this wasn't the case. Not only was it still lwhere it had fallen from my finger, I was actually able to spot it, lying in the gutter outside my favourite gaming shop.
Possibly the luckiest find I've made in a long time. And certainly a great relief.