The light came back on this week, probably because I had the CD changer out last weekend and, in the process, disturbed one of the connectors in the boot. I have now located these yellow connectors and they are easy to get to without pulling the boot lining out, being behind the flap you use to access the bulbs. I cleaned both connectors, plus the one under the passenger's seat again and, as I had the battery disconnected, carried out an ECU reset. The light has gone now.
Effects of the ECU reset otherwise are hard to quantify as there wasn't a lot wrong before, but it did no harm that I can determine and, subjectively, the engine may be a little smoother now.