I used a mains tester to ensure I had live feed to the switch. This is also just a recent problem. Everything worked perfectly for the past 40yrs.
Sorry about the delay. The readings are:
Switched off: Live 19.1 Neutral: 22.7
Switched on: Live 0.05 Neutral: 22.7
I don't understand the readings from a meter, so I have no idea what they mean.