D'oh!
I booked today as holiday, so I could be at home for someone to come and replace the gas meter.
I've only just remembered this, after working most of an uncommonly productive day. Oops. Hopefully HR will be nice enough to let me have my day of holiday back, and I suppose I'll have to apologise and reschedule the gas meter people.
This isn't the first time I've accidentally come to work when I was supposed to be on holiday. The other time was a few years ago, when I booked an afternoon off to go to a garden party, but forgot to go home at lunchtime. What seems to happen is that it simply doesn't occur to me to wonder if today is anything out of the ordinary; I just get up, do the morning routine, go in to work and sit down. It's much easier when I take the whole week off, because I'll have disabled my alarm on the previous Friday morning so I can tell it isn't a normal Monday when I don't get unwillingly dragged out of bed…