OCTAVIA LIGHT RECOGNITION SENSOR

My Mk3 VRS has a double fault with the lights. First is the headlight(s) (and tail lights) come on when I start the car - even in bright sunshine. Sometimes they will switch off after a few minutes. Secondly, the N/S headlamp doesn't come on at all (master switch in manual or auto positions) - but - occasionally it does, for example when I switch off the engine at night and the get-you-home lights turn on, both headlamps work (this doesn't happen every time). Obviously I have fitted a new Xenon bulb and nothing has been disturbed - no new screen etc.

Anyone have ideas where to look?