The filler neck/tube is simply a push fit into the tank and is seled only by a fat rubber dough-nut style gasket. With everything disconnected at other end of filler tube including the built-in vent tube (push fit again in top of tank) very carefully rotate filler tube left and right and slowly pull out of tank. Might be an idea to spray a little WD40 to help get it moving.
My tips would be;
1) Clean around every tube entry point to avoid getting dirt in the tank (unless of course you're changing tank)
2) If the rubber gaskets show signs of splits/deformation replace them -it would be very annoying to get it back in and it still leaks!
3) As a lubricant for the gaskets when fitting them and the tubes into them only use diesel fuel (for a diesel T25!) as then you are sure you haven't contaminated the gaskets.
4) If the throttle cable shows signs of fraying at either end, replace it now as it's a lot easier with the tank out.
5) Check all exposed cross-members for years of mud accumalation and clean it all up!
6) Replace fuel filter within a few weeks after fitting new or old tank as there will have been some debris in there from somewhere!
Job well done!
