Thanks Aerials (and all the others who have taken the time to reply)...I like the cut of your jib (sp?).
More years ago than I care to count I used to compass swing Canberras out on RAF Wyton's compass bay. It used to take hours. If it went well. I recall complex graphs with all sorts of primary and secondary errors to be quantified and compensated for.
For now I think I'll stick with what the aeroplane's kitted out with and try and identify if one of the compasses is any better than the other, if either can be adjusted and why there are two in the first place.
Thanks again.