Neil,
As I remember of the 3 ex-Fin navs on 230 in the early 90s one moaned A LOT (and still does), one was a knobber (who had been booted from Hercs to Finn to Fin to us) and one was cheerful and soft spoken (and bald). Guess who the crewmolds liked...
TR - I believe the value of a spec aircrew navigator depends on:
-the crustiness (or lack thereof)
-any chips (one on each shoulder is preferred)
-how much they smell
and of course the most highly prized are NOT the perperual motion variety (ie they dont go on and on...)
Any other tips I've missed?