Can anyone suggest what I might do to diagnose and eventually solve the following problem:

My 1981 300TD stops running when the tank gets to a little under 1/4 full according to the gauge. If the problem were just the gauge I wouldn't be concerned, but removing the drain plug at this point and a good 10 or 15 litres of fuel came out, indicating that the gauge is correct. This means I'm losing about 100km range on my tank which is not insignificant doing country driving here in Australia.

I suspected/hoped that the fuel delivery and return lines had been swapped (the person I bought it from had it serviced pretty regularly but not by a Merc specialist) but have confirmed that this is not the case. I'm assuming the problem is with the tank, but maybe it could be further up the fuel line? Though that seems pretty unlikely as the fuel gauge reading is very consistent when it happens. Other information: I'm running it on veg oil, but the behaviour began when it had only ever been driven on Diesel, and still happens exactly the same when it is running on Diesel.

I'm hesitant to drop the tank out of the car, given the reports of how difficult a job this is, and having no guarantee that it will accomplish anything.

Any suggestions would be greatly appreciated!