Indeed the problem was the heater in the fuel filter assembly. It sounds like at first it appeared to be OK but on further testing a fault was found ("traced fault to heater thermistor open internally"). The fuel heater turns on with the glow plugs and I guess rectifies fuel gelling that can occur. The heater apparently can be taken out of the fuel filter assembly but the entire assembly itself was the part they had to order. In any case, everything they did before that made no difference at all and with the new part the truck starts perfectly well even on a day like today (-8 Celcius). I like my truck again. Hope this is helpful.