Good post Helmet Fire. Although this thread is concerned with UK AA flights it is useful to remind people that night AA flights have been operating successfully in Australia for well over 30 years now.
Having experience flying AA night flights in Australia and UK for me the main differences were attitude and currency. You have a (comparitively) very pro-aviation CASA in Australia which helps enormously and the prevailing attitude is that flown properly with correct rules and aids there is little risk with night AA flights.
Australian AA flights are (as HF said) up to 60% of all tasks and when flying 850 hours per annum that is a lot of night flying including off-airport landings. The equates to very current aircrew in night operations and night currency training flights tend to get in the way of real night tasks. However training is still important and regularly flown. UK AA flights fly much less than this and a tiny amount of night flying so night AA is seen by many as a very difficult and hazardous.
There have been night AA accidents in Australia in the past but these have been with VFR single-engine machines operated by organisations with poor CRM and flown by pilots who were uncurrent and/or lacking in decision making skills.
Australian HEMS are (mostly) single-pilot IFR machines of the Bell412/AW139 size. The Australian model means that it is possible to fly with a crew of pilot, crewman, rescue crewman (all using NVG) doctor and paramedic and carry two stretcher casualties. The UK model tends to utilise smaller machines of the EC135/MD900 class with pilot and 2 paramedics.
The two countries are very different in many respects but Australia has shown that night AA tasks can be performed just as easily as day tasks with the right aircraft, crew combination, night vision equipment, currency, rules, regulations and attitude.