Thanks for the reply. All was OK, and rode the bike home after checking the voltage across the battery. voltage was around 7v. Assumed it’s the rubbish bulb holder, and have left it alone – something to investigate on a rainy day.
Maplin was just across the road, so bought the cheapest one they had. Now it lives in my toolbox.
If I had been at home, I would’ve used my normal, analogue voltmeter – it’s easier to visualise what’s wrong when there’s a dial involved!