The RPI figure used by NATS for the last 2 pay rises in Jan 07 and 08 was the official RPI figure from the previous August as agreed in the pay deal.
I am sure the RPI that would be used if the deal is sanctioned would be the "official" figure that is used currently by CAAPS to increase current pensions.
In the glossy brochure, it does state "the Retail Prices Index" so I guess they will have to use the figure published by the government.