The CAA safety directive is only 2 pages in not very detailed - it makes reference to conducting HEMS using NVIS iaw JAR-OPS 3005.(j) which simply states
[(j) Night VFR operations with the aid of
Night Vision Imaging Systems (NVIS) shall only
be conducted in accordance with JAR-OPS 3 and
procedures contained in the Operations Manual
for which a specific approval is required.]
JAR-OPS 3 gives a minimum cloud base of 1200 ft and 2500m min vis for night HEMS and talks about requiring any operating sites to be illuminated (by the helicopter) sufficiently to identify obstacles. It doesn't seem to specify if both pilots (on a 2-pilot crew) or the other crew member (on single pilot ops) both need to be using NVIS and there doesn't appear to be a minimum light level for NVIS ops dictated.
One requirement for night HEMS pilots is that they complete 30 mins IF (either in the aircraft or in a synthetic trainer) in the preceding 6 months - I do hope no operator would stick to such a ridiculously low requirement. IIMC on NVG can be very disorientating so your IF skills need to be polished and 30 mins in 6 months just isn't enough.
Fortunately the majority of HEMs pilots seem to be ex-military so the NVIS skill set will already be there but single pilot NVIS ops is going to be testing.