Brian
This was covered in great detail in the original thread over a year ago, including diagrams of the fuel system.
Because the failure of an inboard engine on a 747 is fairly innocuous, crews have always been tested on the simulator by the failure of an outboard engine. They are therefore used to dealing with the subsequent imbalance between tanks 1 and 4. However, on this occasion the failure of No 2 engine meant that the crew had to use the Override/Jettison pumps to utilise the fuel from tank 2. When the crew diverted to Manchester, they were planning to arrive with more than the required reserve fuel. However, on the final approach a FUEL OVRD 2 FWD EICAS message appeared, and in the mistaken belief that all of the fuel in tank 2 might be unusable they took the prudent step of declaring a 'Mayday' to ensure that the runway was clear.
As all of us who have have had time to analyse this at their leisure will of course know, the Override/Jettison pumps have a stackpipe which prevents them pumping any more fuel when the tank quantity is 3,200kgs. This is to prevent the inadvertent dumping of all of the fuel. The normal fuel pumps would however have continued to pump the remaining 3,200kgs of fuel in this tank via the crossfeed valves. The aircraft actually landed with more than the required reserve fuel, all of which, with the benefit of hindsight, was usable.
One of the nice things about aviation is that we always learn from previous errors and mistakes. On our present recurrent simulator check we cover all of the fuel system problems in detail. As part of this training we replicate the exact situation the the BA crew found themself in.
I hope that this answers your question.
Airclues