Thanks, that is quite reassuring. The "sludge" is very sticky - I only noticed it when I went to lift the gauge rod up and I could feel it sticking to the bottom of the tank. What I found on the end was some dark brown very sticky residue. I have never had a problem with dirty or watery fuel, so I do believe that what I am seeing is not an urgency. It doesn't strike me as a DIY job though, so as you suggest, next time we're in a reasonably up to date location, we'll call an expert.
Ya Fohi - Amel 54 #98