Art - many thanks, understand from you that XFER pumps are not operated until main tank fuel is very low, ie that there is gravity feed for most of the flight. This has not been clear to me from previous explanations. The diagrams do not show details such as pump mechanisms nor air vents.
I can confirm that I followed all of the first third of this thread (until the repetitions became boring) and all of this thread since the AAIB Special Bulletin. My interest stems from experience of avionic design, including mechanical items and black boxes, for example autopilot and FADECs.
Edit - okay reading and digesting Art's 10:25 edit