The air ambulance is mainly operated during daylight hours to call outs, though transfers take place during the hours of darkness (mainly SAR territory), so the 'net' of fuel stops at night in Scotland will not have developed much if at all beyond the airfields that have regular profit making hours.
The RAF/RN/CG helicopters have their own fuel supplies around the Highlands located at some of their regular landing sites. They are 'self-service'.