OK. I did it.
Took battery out of the car. No power tools, no torches/phones, doors open & outside.
Remove the back seat (2 bolts rear, 2 bolts front), undid the cover plate (3 catches), removed the pump/sender connector, unclipped the fuel pipe.
The lock ring is a total b******. I cleaned all the grot off it, hoovered out all the debris and tried to budge it. No chance. Note there is a white dot on the chassis and an arrow on the ring indicating how tight it needs to be... Solid. Solid as a rock (I feel a song coming on).
Soaked the joint in WD40. Went and had a cup of tea. And some cake.
Came back after 20 minutes. No change. Lots of ridges on the ring, so got a BIG screwdriver and a medium sized persuader (hammer) and spent the next 30 minutes tapping all round the ring until it eventually started to move. There are only 900 degrees/2.5 turns until it is undone.
Got the filter/pump/sender unit out. Siphoned out the fuel that was left (about 5ltrs). Remaining fuel removed with clean sponge until dry. Then used micro-fibre cloth to clean any remaining specs of crud. Tank now clean.
Allegedly, the filter/pump/sender is not user-serviceable, but there are three catches which can be gently pried off to get to the filter. See attached photos. These were cleaned and rinsed, then reassembled. Note that the filter can be gently removed from the main body of the unit to ease cleaning...
Getting the unit back in was a complete b****. The unit is spring-loaded and must be held down firmly and then the green gasket slipped under the rim - if its allowed to pop up even slightly, the gasket falls into the tank and you start again. And again. And again... Two man job - one person holds the unit down so the gasket stays in place, whilst the other desperately tries to get the lock ring to engage and tighten.
It took us an hour, and we're both professional engineers (not motoring ones, unfortunately). We had to have more tea half way through. And cake.
Put some clean fuel in it. Car now runs smoothly, but not taken it for a full road test yet.
One slight question - there is pressure in the fuel tank - after I've run it for a few minutes, if I undo the fuel filler cap, there is a fair old bit of pressure there. Pshhhhhhhhhh......
Is that normal? I don't remember that before.