Have been happy to endorse the technical support before but now for the outcome Mine is a 60litre tank in a 29’ sloop tank hadn’t been used for some time. (Had been using a plastic tank instead for some years).
Recommissioning the main tank  I had drained it and tried to filter the fuel with an external filter and pump but was still getting some muck  and greasy sludge on hoses when I used them.
Put three litres of fuel doctor in to 30l of diesel and left it for a few weeks. The result was dark brown fuel which still ran. Given how little fuel I burn.  I emptied again and used the same three litres to
30 litre mixture. Result after a couple of weeks this time was clean fuel and a clean looking tank.  Short answer is that the fuel doctor additive worked  very well in a pretty far gone tank.
No paid endorsement no discount offers just a good outcome and a great lot of help and support along the way. Thanks one and all.