Thanks firstly to all who chipped in with advice on what might be wrong with my lights and immobilizer keypad.
As it turns out they both receive their power from the same wire which had rotted through just under the radiator. I am not surprised it rotted as the insulator (white) looks like something from a 1950's Rowenta iron.
Anyhoo, she's running again and is actually starting more quickly which makes me think that the wire may also be conntected to the pre-heaters somewhere. With a Xantia I could believe anything.
Cheers