As previously mentioned, the onus is on YOU to fill in a tax form once you are into 40% tax.
It would be helpful of whoever is paying you to tell you when you reach this point, then you will not be caught out as some have been.
The IR can send anyone they like a self-assessment form, so they have you every way.
If you file electronically (I did last year), all you will get in subsequent years is a reminder to do that again, and if you wish to revert to the paper version, you are required to phone them and ask.
The tax office that deal with NATS staff have been very helpful on all occasions that I have contacted them.