It is level 5, level 4 is the minimum but (this is all of the top of my head, but I'm sure you can look it up in the docs if you'd like) ICAO also says that units that require a higher level may stipulate a level 5 is required, NATS has as I've understood it decided that all their units are complex enough (maybe due to the amount of naitive English speakers) that everyone requires a level 5 or higher.