Okay SyEng, I like the way you argue your hypotheses.
There is another point worth mentioning, perhaps, again quoting the AAIB:
During the descent, from FL400, the A/C entered the hold at Lambourne at FL110; it remained in the hold for approximately 5 minutes, during which time it descended to FL90.
Entering the hold at FL110 is a completely different regime from the early part of the descent that I was dealing with above. The aeroplane will be on speeds now, not Mach. Entering the hold, the IAS would be back to about 210 - 240 kts, to comply with holding-pattern rules.
Also, although they continued to descend, they only averaged 400 ft/min during those 5 minutes. This is minimal - not much different from level flight. The pitch attitude to fly at (say) 220 kts would have been of the order of 5 - 8 degrees nose-up. This is as much or more than on final approach.
Additionally, there is every probability that significant thrust would have been used, unless they entered the hold at a higher speed than optimum, and were slowing down.
I wish the AAIB had told us whether thrust above idle was used in the hold, or later on the intermediate approach. It normally is at LHR.
You will draw your own inferences. A B777 pilot could tell us how much fuel-temperature rise normally takes place between FL400 and FL90 in a typical descent, with about 12 tonnes of fuel remaining in the wing tanks. Would it have risen from the -34C quoted to above 0C?
But as for your postulated large block of ice in the centre tank, who can tell?