Air 27/3579 (the 50 Sqn 540/541) offers some insights, but confuses as well as helps if taken at face value.
For Black Bucks 4, 5 and 6 the crew (spelling below is as it is in the document, BTW) list is:
Sqn Ldr CN McDougall (Captain)
Flt Lt Gardner (co-pilot)
Fg Off CCM Lackman (3rd pilot)
Flt Lt DA Castle (Radar)
Flt Lt B Smith (Plotter)
Flt Lt R Trevaskus (AEO)
Annex C of the 540 for May 82 describes BB4, and describing the start of the sortie, the author (unnamed, but I presume Flt Lt Gardner) says 'As AARI, I was flying from the right hand seat' - I assume that this was Flt Flt Gardner, but cannot be certain.
For Black Buck 5, the crew is listed as above, minus Flt Lt Smith. Since he is mentioned in Annex D of the 540 as being part of the BB5 crew, as well as being name-checked in Dave Castle's chapter in Vulcan Boys, I'm assuming that this was an omission by whoever complied the 540...
And for Black Buck 6, the same crew again - and again, Flt Lt Gardiner [sic, note changed spelling] is described in Annex A of the 540 for June - as the AARI.
Black Buck 7 was flown by Martin Withers' crew, of course.
Flt Lt Clifford is listed as being the 6th crew member of Sqn Ldr Montgomery's crew.
And finally, but by no means least, Flt Lt Gausden is listed as the 6th member of the McDougall crew by the 540, but as we have seen, Flt Lt Gardner was in that position for BBs 4,5 and 6. I think I am correct in saying that he was the Ppruner 'Art Field', who is sadly no longer with us?