Hi Robin, been reading this thread and it is becoming very clear you are quickly coming up to speed with the salty way of life. I am very surprised a company such as Trader have installed mild steel fuel tanks, I really thought these were a thing of the past. As others have said it is a good decision to get a professional fuel polishing company in. When we bought Eighth Day Sealine S42 we were very lucky that the previous owner had had all three tanks cleaned and fuel polished a year before our ownership. When this was carried out they cut several large inspection holes in the top of each tank in between the baffles to ensure all crud was removed.
Earlier this year I was talking to a guy from a fuel polishing company who was cleaning fuel and tanks on a large boat in our marina in Ipswich, he told me the biggest problem with diesel in the boating world today is biocide diesel due to degrading within a time scale as short as six months. (Much the same as petrol). Biocide desiel is fine in cars and trucks on the road as they tend to have high turn over of fuel, however with boats, fuel sits in the tanks for months, even years without being used. Apparently, this is more of an issue with modern comman rail Diesel engines with electronic management, sensors read the quality of fuel and if not satisfactory they go into limp home mode.
Unfortunately, many marinas are now buying in biocide fuels, luckily where we moored at ipswich they were aware of this issue and only brought in non biocide desiel. Although they did say it is now becoming increasingly difficult to obtain.