In response to the above, yes experience does count for a lot. I seem to remember that one of the oil companies insisted that we carry out 3 take-offs and landings at night every 90 days, until we pointed out that just couldn't be done from April through September in Scotland, and they backed down !! Crews used to shuttle in the Brent many years ago and found they became far more confident and proficient at night flying - Still that 1st flight after 2 weeks off or the first week when the nights came in in October were still a bit of a shock !!
I agree that high latitudes do present a problem, but not a huge one. Even at the very worst part of winter, there is daylight from about 08:00 to 16:00, slightly less way up north in the Shetlands (if I remember rightly, its about 08:30 to 15:30) and is only this bad for about 3 weeks. I've always felt that flying at night is fine, it is the deck operations, shuttling etc that pose the greatest risk. Imposing a curfew on night decks still provides that additional 30 minutes (minimum) either side of night to allow the aircraft to take off at night and arrive at dawn, or depart the deck at dusk and arrive back at the airport at night. So, even in those shortest days in December, there are at least 8 good flying hours to get the work done - Yes, you miss probably a whole rotation in the afternoon, but this is public transport we're talking about, you don't need those flights - For 2 months in the winter, you could easily put on more flights at the weekend and I think safety would be improved.
Interesting to note the US Navy do not allow pax flights at night over water. I worked in Africa for a while where a number of oil companies there seemed to have recognised the risk and will not allow helicopter flights at night, whilst we here in the UK don't seem to have made the connection. Perhaps it is because at least 2 of the helicopter operators (certainly historically and I assume still) offer cheaper slots later in the day.