OK I gave up on access through the float switch hole. Thanks to another item on this site, I removed the pick up the Eberpacher pick up fitting and wiggled it around and hey presto I'm in.
Syphoned out a bit of fuel from the very bottom of the tank and got less than a cupful of water and grot.
So peace of mind; however as is my situation everytime I work with tubes and diesel, I manage to make a mess.
Pass me the Mr Muscle and some paper towel!