@samblundell
Now this document is 4 yers old so there might be a newer one, but this was the one I found with a quick google.
For refrence it can be found here
http://natscareers.co.uk/PDFs/NATS_A...lClearance.pdf
and my quote is off page 9.
Colour Perception
Normal colour perception is defined as the ability to pass the Ishihara test or to pass Nagel’s anomaloscope as a normal
trichromate .
An initial applicant with less than perfect colour vision shall be classed as unfit.
I'm not sure to what degree you are colour blind, but if you haven't taken any of those tests maybe you should try it out before you spend a lot of time and money on the application process?
Sorry to give you bad news, but I suppose it would've turned up sooner or later...
As for the pass mark NATS doesn't say anything about each test, but their website states that 3% pass stages 1,2 and 3 put together. But as for each test, your guess is as good as mine