Thanks both. Longmoor is a bit restricted from the railway point of view, unfortunately, so not for me, I'm afraid. I remember from childhood stations with 3T GS and sometimes other vehicles waiting outside for National Service arrivals or people returning from leave, or to collect stores, and have noticed from contemporary photos how often military, or ex-military, vehicles appeared on railway vehicles. The railway seems to have carried whole trainloads of identical vehicles on occasion [presumably vehicles being returned to stores or their replacements, vehicles being taken to a disposal site or vehicles issued to TA units for exercises], but this is not really practical in model terms. It also sometimes carried smaller numbers [to/from TA units?] and unregistered individual vehicles, presumably from a disposal site, either of which is much more achievable.
Census numbers had disappeared by the period I'm interested in, replaced by 1949 series numbers. Vehicles from BA [armour] and BC [softskin] were, so far as I know, new vehicles, though, and never carried census numbers. Anything originally allocated a census number would have been in the Rx, Xx, Yx or Zx series, like the very first batch of Land Rovers for the army, ordered in 1948, which were in 90YJxx.