Fine if you can get the tank out and take it to them. I did the tank in my Sadler a few years ago in-situ by pumping it out then vacuuming out the remains of the diesel then steaming it with the aid of a lance made from a piece of copper tube (3/8" ø) fed from a pressure cooker on a gas ring. A bit "Heath Robinson" but it did the job - only problem was getting the emulsion out of the tank though a squirt of washing up liquid into the tank helped keep the oil sludge in suspension in the condensate until it was deep enough to pump out into some old drums. Plenty of rags and a pair of welder's gauntlets definitely required to prevent burnt hands.
Well, chaps, I am not as daft as some would apparently think. I am always amazed at how often on this forum one is advised to refer to the professionals ( or proffesionals (sic) to quote a recent one). I think I will remove the inspection plate, turn it upside down and contrive a steam supply. I asume the heat and moisture softens the gunge and muck so that it dribbles out. Has anyone out there actually done this? How do I know it's completed? How long does ir take?