Ian wrote:
Thanks for all your replies - the problem is now fixed.
Having removed the old light, which was working fine, and replaced with a
new light, which wasn't, the solution was a new switch.
Why it should have failed in the short time taken to replace the light, I
don't know. Does anyone have any suggestions?

You shorted the switch across the live and neutral (remember the MCB
tripped). You killed the switch.

Owain