This was going to be a slow process. The pump seemed to produce about one litre per minute and there were those 130 litres of diesel in the tank. I didn't actually need to run every single drop through the system, but I did want to try to get at least the worst out of the bottom of the tank. I did try gently rocking the boat to get the water layer to move around and get sucked up, and pouring the 'polished' diesel back into the tank will have helped to stir things up a bit. I lost count of the number of times that I drained off the water layer.
But the engine would not re-start! I tried using the priming pump on the top of the Beta Marine's fuel filter housing, but it seemed unable to spring back up and draw in fuel. Did I now have a defective filter housing as well? So I removed the new filter and dismantled the filter housing to check: No, the priming pump was OK and the spring was working. So there must be a blockage in the fuel line.
. . . to reveal this grotesquely clogged filter! Hardly surprising that no fuel was getting through! How on earth had ANY fuel got through for the last few days / weeks / months ?
It looked like I had found the true culprit, so I reassembled the water trap without any filter. This was the first time that I had opened the water separator since the boat was built in 2007. I had forgotten (if I ever knew) that there was a filter inside. No doubt it said so in the small print – which I will have read 16 years ago!
So, after some more lengthy bleeding out of air in the fuel line, the engine re-started successfully and was its customary responsive self, accelerating and decelerating as normal. With no filter in the water separator, any gunge was going straight to the engine's fuel filter, but I had already reckoned on that new filter element getting clogged up quite fast with bits from the fuel line, so that was OK as a temporary solution. I had a few spare elements ready!