Could be simple like belt slippage (would probably hear it squeal though)
Check the wiki there is bound to be info there, or do a search and someone would have posted same problem with a possible cure. Hope it somthing simple
Well I have to drive her back from Dorset to Cambridge at the weekend! The belt used to slip and squel (light came on and went off when reved) but was tightened. the light is VERY weak and flickres, you have to shield the light aperture to see it Cause for concern, it is over 200 miles?
I had a similar problem. When you revved the engine the light went out, however when ticking over the light would be dull...And i'm afraid to say it was the alternator that was on its way out..
If belt is old, tightening may give temporary cure but you put strain on alternator. Belts vary a lot in quality I find. Pulley wheels can end up polished like chrome with a slippy belt and ofcourse that merely adds to prob.