Thanks guys. This came up in a CAE built sim the other day. Inserted DC1+2 Fault. Status said 'Outer Tank Fuel unuseable' so I was wondering how, if you were up against it, you might get that 1.5T of gas back. I agree that the FCOM for the aircraft I am involved with do not reflect that. Must be a sim thing!
It appears from the helpful answers above that you do need electrical power to motor the transfer valves open and closed.
Cheers
mcdhu