Main beam warning light intermittent

My blue main beam warning light on the dash is intermittent - main beams (xenon) come on fine but increasingly there's no warning light to accompany them, just the normal green headlights-on light. I've just noticed as well (though it may be unconnected) that the indicator for manual main beam on the rotary switch doesn't light up (the small green light with a picture of the main beam), though I'm sure it used to. I gather the main beam warning light is now an MoT failure so I need to do something about it. Any idea what the problem might be?