Thanks - it is the bulb in the housing... will investigate later (one of the few jobs I can do today and stay dry).
The Haynes manual is pretty rubbish - it shows the exploded view of the selector mechanism - but no bulb at all!
Got access to the bulb - but it was OK. Checked the bulb holder and there's a break in the circuit somewhere... godknows where.
Think it's a job for Super Old Renault Specialist (whenever I can find one...)