SI Clik, thank you. My source confirms that the RAF P & N tests are unrelated. True a pass in both shows a flow rate training risk and a P/F shows a higher risk.
Provided there are insufficient P/P candidates then I think the system would accept a P pass and an N fail into training but, as I said earlier, the probablity of a further career after failing P trg would be remote and aircrew remote to vanishing point.
Bottom line though is nothing ventured nothing gained and the RN Observer aptitude tests are quite different from the RAF.