Thanks for your thoughts guys.
Cough - TAT probes could be the answer ....not sure how much air they use...the hyd and water would be minimal unless someone locked the water tap on. Is aux fuel tank pressure BBJ or am I missing something obvious.
BOAC - Pls explain
FCS - My initial thought was use of APU could be in there somewhere...but it goes against the QRH..and we can't use the APU for wing anti ice. Also...I suspect the FMC output would not be allowed to make an assumption that APU was avaiable and/or selected.