Thanks again all, I am really grateful this forum still exists, and that so many of the old school players are still here.
On your recommendations, I bought three cheap Fram fuel filters for the project.
Then I pulled out the T fitting for the hoses leading from the vapour tanks to the charcoal cannister, and popped a fuel filter into each of the two lines leading to the vapour tanks. Charcoal cannister is now redundant, and I will pull it out when I get time.
I chopped one nipple end of two the Fram filters off, and then drilled them out to 3/16", to better match the I.D. of the hose from the vapour tank.
So, job done. (Well that, plus the new fuel tank, which is a b!tch of a job with getting the crossover tube and hoses into place in a 2wd Vanagon, I can assure you. Probably had that thing up and down six times, with trial and error running the crossover and vent hoses.)
So, with a new Bosch spin on fuel filter filled up with fresh diesel, and about 15 litres in the new tank, I did a test start.
The engine flashed right up, and ran for a few seconds until the diesel in the fuel filter was exhausted, then it sputtered out to a stall.
Finally, I pulled off the aftermarket check valve, that was plumbed right before the filter.
I replaced that check valve with the third of my Fram filters, tried to start it again, and all was perfect.
On testing the check valve on the bench, it was clearly blocked.
So I think that check valve was the key cause of my initial shut-down, air bubbles in the feed, the milkshake in the return line, and the odd suction reversing from the tank up the return line into the injection pump.
I notice that the new Fram filter drains about half way a few minutes after engine shut down. I guess this was the symptom that caused the previous owner to install the check valve in the first place.
Not sure whether it's worth installing another check valve, or whether I should just live with a little air in the lines on cold start.
Thoughts?