Looks like I am buying another NW chainring (and possibly chain) and seeing if that fixes the issue. Seems a sensible option. Thanks for the advice.
It did start dropping (like once or twice) at the latter end of a week in the Alps, where on some days I was doing a reasonable amount of cranking up mountains (They totally need more chairlifts) – so it is a reasonable assumption that something (either the chainring or chain) got worn.
@tpbiker
With a hardtail IMHO yes. Until that point I didn’t have a problem with it dropping (and it still doesn’t drop that much, just enough to be slightly concerned when racing) – I did run a bash as well but I don’t think it did much/anything to help.