Accretion.
IMHO I can't help but think that we are still missing the point.
Accepting that sufficient evidence exists to point to fuel restriction I remain open minded regarding the cause.
For ice to have formed, to the extent required to restrict flow, a considerable amount of water would need to be present in the supply from the fuel tanks and in addition some mechanism would have to be present to make this situation common to both LH and RH systems.
Now the AAIB choose the word "accretion" I believe with considerable care. I suspect that used this word since it infers the presence of a "nucleation" site. It also indicates "growth".
So a mechanism has been proposed that ostensibly could be common LH to RH. However the conditions by which the growth takes place has not been indicated. i.e. Growth from passing ice crystals "aggregation" or "freezing" of dissolved water from solution.
Either way COULD explain why such a RELATIVELY small quantity of water caused the restriction since the restriction would build up over time.
IMHO I wonder whether the intuitive design practice to consideravly oversize fuel supply lines is in fact counter productive since this reduces fuel flow rates which ARE LIKELY to significantly accretion rate.
Add to this the fact that fuel economy is improving and thus reducing flow rates MAYBE we have reached the reached the edge of the envelope of our (lack of) understanding of permissible water content.
This might actually support why the AAIB actually only sited the Trent 800 and left the wider picture to other agencies.
Strikes me we are facing a design maturity / standards issue and that it is an issue of lack of understanding (technology maturity) rather than something specific being/having gone wrong.
Following this logic I would have have thought that some relatively simple experiments could be done to determine accretion rates at differing water concentrations, flow rates and temperatures, could be produced.
The difficult bit will be determining whether the subject aircraft meets these "new" requirements and then subsequently potentially all aircraft in operation.