Whippersnapper,
The AAIB's theory is that ice had (probably) adhered to the walls of the fuel feed pipes in the wings and was released during the descent (because of the increase in temperature) causing a restriction in fuel flow at the FOHE. They have been unable to simulate the operational environmental conditions to cause a rollback, except by shoveling lots of ice into their test rig just before the FOHE. There is doubt that the amount of water believed to be in the tanks of G-YMMM could produce the amount of ice required to restrict the fuel flow in this manner. This is a simple precis of part of the AAIB's findings so far, and is an inadequate substitute for their report. Their investigations continue.