And yet another update to this saga I thought was dead and buried.
I was on the road a week or so ago and experienced a rather sudden and very drastic loss of power, even more so than the previous time. So much so that I decided to head straight back home. Jumping to conclusions (which is often the mother of all stuff-ups) I figured the most likely culprit was another fuel blockage. Although eventually being a valid guess, I really should have used my brains are connected the scan tool first, as it might have pointed to something else altogether.
Anyway, despite having previously given the fuel a "shock" dose of anti-bacterial additive, I emptied and removed the tank for close inspection, fully expecting to see another build up of bacterial growth ("algae"). I was pleasantly surprised to see no sign of it. I next tied a white cloth around the tank end of the fuel line, disconnected the filter end and blasted it with air. The cloth showed no contaminants blown out, so the line was clear. I reinstalled the tank and, while giving my back a chance to recover a bit, changed the fuel filter.
With the new filter in place the car was back to normal, so after the test drive I cut open the old filter and pulled out the element. It was completely covered with what looked like the remains of the bacterial growth, which had been killed by the treatment. It was so bad that when I tried to pass diesel through just a single layer of the element it barely got through at all.
Lessons learned: After using such an additive for treating diesel bacterial growth replace the filter soon afterwards. I'll also be changing out the current one within the next 1,000km as well, so to be sure it's all gone.
Although it turned out that emptying and removing the tank proved unnecessary, I do have the peace of mind provided by knowing I appear to have solved the problem of what I had at one point thought of simply as "black sludge".