Spaz, That's a very old fuel system schematic, from the days when there were 8 individual tanks rather than 4 pairs of masters and slaves. I think that probably only applied to some of the early development batch aircraft. Even so, failure of a single jettison valve to close should only lose half the fuel in the aircraft at most. I suspect there is more to the cause of the accident than is apparent from the summary reports.
In the 1970's we modified a Buccaneer to carry out Foxhunter radar trials and fitted a fuel recirculation system which took fuel from the jettison lines through a heat exchanger and back into the refuel gallery to cool the radar. I think the aircraft was XX897 and we knew it as the RRE Buccaneer. I did all the calculations on flow rates, pressure drops and temperature effects and a colleague of mine, Eric Lewis defined the fuel system mods and controls. Our boss at the time was a chap called Ray Gadd who had been a Halifax gunner during WW2 and was shot down returning from a raid on Germany. They ran out of accessible fuel, i.e. there was still sufficient fuel in the aircraft to reach the UK but it was not accessible to the engines that were still running. As a result the whole crew parachuted into the channel and were captured and spent the rest of the war as POW. (the actor Denholm Elliott was also part of the crew) Ray had been involved in Buccaneer flight systems design from day 1 and as a result of his experience in the Halifax, was fiercely protective of the fuel system redundancy and fault tolerance. Eric and I spent a very long time in discussions with Ray before we persuaded him that he could sign off the mod on behalf of Mechanical Systems. Mike Brooke describes his experiences operating XX897 in his book Trials and Errors.