There's no particular right or wrong time to get the NQ done, as long as it's done before CPL/IR skills test. Winter does give you advantages though as mentioned by other posters when it comes to airfield opening times - having had a few late nights earlier in the year at Coventry getting people done in time for CPL courses!
I'd tend to agree with MJ about the survival rates for night incidents in singles, especially knowing a (very) little about the case mentioned. Some instructors even suggest pitching to Vne and waiting in the event of an engine failure, it makes the waiting go faster... Terrain especially has to be considered, bearing in mind that night flight involves complying with IFR - I do get a little worried sometimes hearing aircraft transiting around below 2000' at night round the edge of built up areas, over unknown terrain, etc!