Removed the access plate to the fuel tank to clean out any sediment etc that might have been lurking in there... infact it was better than I thought with just a small layer of crap on base of the rubber bag. The fuel sender / float assembly is a big bugger !!
I had not noticed before that the little viewing window had a level reading behind it.... was mystified at first as to why the numbers were reversed and upside down !! .... but I guess as the window is in a position where direct viewing is impossible....there is a small mirror which flips out.... then, through the mirror, the numbers look right... very clever !
Also the numbers show how many litres are required to fill the tank.... not how much is left in.... also very sensible.