Just one other thought - depending on when it was, could it have been a Sunday working on the 53 rather than a 122A?
The 53 replaced the 122A on Sundays from 1956, although initially from Old Kent Road rather than New Cross (until after the 'pre war' RTs had been withdrawn from passenger service) - I'm not sure if Old Kent Road was in the same engineering group as New Cross (being closest geographically didn't necessarily translate in to groupings that worked on whatever basis it was done on) - although again seems unlikely that a loaned non-standard bus would have been used on a Sunday.
The
detail on Ian's Bus Stop website (which the site owner makes clear is not 100% complete) shows Cravens RTs allocated at Old Kent Road, New Cross and Catford garages (all of which had routes that ran to / through Eltham), but not Sidcup, Abbey Wood, Plumstead or Bromley (and they had been withdrawn from service before Bexleyheath got buses) - would have thought it would have made more sense to concentrate them at one or two garages like the country area did, as almost everything other than the light bulbs must have been non-standard.