So my husband came home and pointed out that we already knew that the car's spark plugs were about due for replacement. Probably the warning light meant that we were using petrol like a Hummer - clearly not good, but I could get to work OK. He poked around under the bonnet, tweaked the spark plugs, and disconnected the battery and reconnected it. The warning light went off, and stayed off throughout a short test drive.
I went to the observatory as planned, and the car was fine. It didn't even use any more petrol than usual - not so as you'd notice.
All that worry for nothing.