The tank I had, not got the vehicle now, had been treated with 'petseal' or something similar. This stuff started to come off in tiny bits which clogged the fuel line and filter regularly. A pal of mine suggested that it was the octane booster I'd put in with the fuel that had lifted the seal. I imagine red oxide would lift with the action of the petrol alone. Anyhow, not wanting to have to strip the vehicle to remove the tank I got a length of garden hose with a lump of rag tied firmly to the end and proceeded to mop the inside of the tank out. The hose quite easily being manipulated and guided to all areas of the tank, even past the baffle plates. I did this through the sender unit hole, as there were too many bends on the filler. The rag(s) being coated in stages with grease, thinners, turps and acetone. Grease being nice and sticky it picked the big bits up and the others I used as I wasnt sure which one would do the 'fine cleaning' the best. I then dried it out with a hot air gun. Same as Jack. It took me a few hours but it was a sunny afternoon and I did have a good supply of cold beer to help with my lubrication. I reckon I shifted it all as the problem didnt reoccur during the rest of the time I had the vehicle.