Trevor.
The two ambulance helicopters operated by the Scottish Ambulance Service from Glasgow and Inverness both operate at night and in IFR (although they only do surveyed sites / hospitals / airports at night).
I understand that at least one of the English charity operated ambulances (Lincolnshire perhaps) has plans to introduce 24 hour "primary" ops with two crew and NVG as soon as they get approval from the CAA.
Someone please correct me if i'm wrong about the second part
OH