i had to drain a full 80 leater tank, on a 200 disco, tryed the hose pipe into the filling hole, but it wouldnt work, if you look under the truck, you can see the big pipe that the fuel goes down to go into the tank, this has a jubaly clip on it. mine was rusted on, so i cut it off.
then put the hose in and sypend it out. then put in a fue leaters of fuel, undo the banjo on the top of the fuel filter and turn it over till the new fuel came through, then chainge the fuel filter, and put the banjo back on. and turn it over and it will start.
took about 30 mins for the tank to drain once the syphon was started, so about an hour in all. but most of that is waiting round for the cans that the old fuel was being sypend into to fill up.