Memory fades, but I thought we had four from BA and two from Pan Am. My recollection is that the original requirement was a tanker that could tank a Herc to Port Stanley from ASI, fail to get in, then tank it back. Of course when MPA was built the requirement became a bit redundant but no-one wanted to hand them back, particularly as the ex-Pan Am aircraft were doing such sterling service flogging up and down the South Atlantic. The ex-BA aircraft went into Marshalls first for a conversion that most people thought at the time wasn't really required, a better option would probably have been just to put centreline hoses on the standard aircraft thus preserving much of the freight capacity underfloor and the full seating capacity in the cabin whilst still leaving a reasonable fuel offload.