Paul. If you wiggle the bulb in the holder there's a fair chance it will light up shes'e getting on a bit and the holders corode, also the wires will be quite brittle as theres no protection from the elements. I did a partial re wire around the lights and horn last year same problem as you but with the horn intermittant as well. while your underneath check the headlamp contraption is cable tied to the body a must on the series1 as they can vibrate and detach from the headlight and are about 180 quid each which I found out to my cost. I think you have a simple fault so good luck fixing it.

Em