212man - yes, my mistake, I wrote N1 when I should have put N2 - it comes from years of calling them Ng and Nf instead of N1 and N2 - doh!
Gulli - thanks, the Bell DECUs and Airbus DECUs clearly work in subtly different ways.
Since many aircraft have the facility to let you beep up Nr for Cat A, either automatically or via a switch/beep, I assumed the 76 was the same.