I was about to head out for a drive in Yoko today and the charging light and 'heat' light on the dash came on. A quick check with a multimeter showed the battery was good with around 13.5V across the terminals. With the engine running it was much higher though, above 18.0V when revving the engine. This is a clear indication that the alternator/regulator has died :-( It is 30 years old though.
I ordered a new one from MX5 Parts. £190 delivered.