pr00ne -
Yes, XV replaced 45 as the plate for the TWCU (as then was) post Granby.
The argument that this should not have happened is based on the notion that 45 is senior to XV , and the senior plate should've been saved, even if it could not subsequently earn 'points' towards its seniority.
XV was disbanded between 31 Dec 1919 and 20 March 1924. It then reformed as a trials unit (my equvialent to SAOEU, but an A&AEE unit would have been a better example), a role it held for ten years before being restored to the front line.
Without getting too long-winded (I'm trying, I'm trying!), if you do the adding up of time regarded as a non-operational squadron 45 is senior to XV by 1 yr, 1 mth and 10 days.
However, the argument rests on whether the time as a Hunter 9 squadron was in an operational role or a reserve role - I have seen material arguing both sides of the case.
In any event, since 45 re-emerged as a plate soon after being displaced by XV, the point is pretty moot - just a question of whether the flying camel should be on GR4s or King Airs. I suspect a King Air marked up as MacRobert's Reply might be a tad inappropriate though!