Today I rechecked the fuel filter. This is what I found (click on the thumbnails for a big pic).


I guess that the paper filled with rust particles then collapsed as the fuel pump continued trying to suck the fuel through. There is rusty sludge in the bottom of the filter. Here is the old fuel filter next to a new one (Halfords HFF201), you can see how much the filter paper has collapsed.

The fuel tank was fitted new last year. Perhaps it had been on the shelf for a while so has some internal rust that is slowly coming out?
Why did the problem show only when the engine is hot? Coincidence? Maybe this problem isn't even related to the intermittent misfire when hot from last October which I thought I'd fixed. The van has been fine on hour-long journeys since then, but this was the first long motorway journey.