Kris wrote:
We live in a small village and have one of the few old mercury lamps
right outside the bedroom.

It used to come on at dusk and go off just after midnight.
Something has gone wrong or been adjusted because it now
stays on until 2.30 am.

Does anyone know how they work ? I assume the switch on is
light sensitive and the off a timer. About 6 in the area seem to be
linked to the same timings.
Can I just ask the council to readjust something before I purchase
some blackout curtains ! ?

If you can find the sensor, and arrange a tripod with a laser pointer
pointed at it, with a small power supply in place of the batteries,
that may work.