Tristar aux tanks
Chips,
I know that in the 70's BA was a 10-11 operator and some of its a/c eventually went to Gulf Air - but I dont think they used aux tanks, although they certainly did the 'two engines on one wing ferry' thing, which is wierd.
Like VC-10s before them, others went to the Royal Air Force for use as transports and (I think) tanker conversions. So I would be willing to bet that the UK and 'MOD surplus' agencies would be a very good place to start looking.
PM