Went out for a spin today and after a few miles my oil light came on. Stopped and listened to the engine - no sound of impending disaster. Tickover sweet (i.e. sounded like a bag of spanners being dropped down a lift shaft - but that's what it's always been like). Switched off engine and checked oil level. Oil level fine (but a little emulsion on the sight glass).
Restarted engine and light went out. It's been fine since.
Going to check oil pressure when I can find an oil pressure gauge, but I suspect a dodgy oil pressure switch (it's happened before).