Thomas Cook are adding a Thursday NCL-CFU flight next year, taking them to 3 per week. It's available to book now and will be a late/night flight. The island is growing in popularity.
Corfu has seen an increase in frequency from NCL in the last couple of years with Easyjet now operating Sundays also. Takes it up to 7 a week for 2018.
TCX - Monday, Thursday Friday
Thomson - Monday, Friday
Easyjet - Sunday
Jet2 - Monday
I'm surprised that Ryanair haven't started NCL-CFU since they operate the route out of just about every other airport multiple times a week, out of MAN they start the route the earliest in the season compared to other operators (March/April) and they usually go out full.