Thursday update: Warning light is (now?) coolant level, not oil pressure. Coolant topped up and all's well again. Was it coolant all along? I'm not sure. The coolant and oil warning light are right next to each other but very different icons. Here's the thing; the warning light came on at journey start, when the engine was cold, so I wouldn't have considered coolant temperature... and I had no idea the car has a coolant level warning (same warning light). Poor excuse though. Next time I'll check the icon and check the manual. You see all the warning lights when you first turn the ignition on, before start-up, and I bet most people don't know what they all are. I do now!!
P.S. Top marks to Fairway Autos of Hatfield Herts, who didn't charge me anything today.
PPS Oil and filter needed changing so no money wasted.