It's like waiting for a taxi on a Friday night out. You never see any, then half a dozen turn up.
Received an auction catalog in the post, and Lo and Behold, a .5-in MkV Vickers for sale. Lithgow SAF Museum doesn't have an example of a Mk V, and none others are known in other collections in Aust Museums or private holdings. Well, I had to do the responsible thing and have submitted an obscenely large postal bid form (well, actually the form is just A4 size).
Just have to wait and see if my idea of obscene is bigger or smaller than someone else's idea of obscene!