In the following conversation it became obvious that Gary wasn’t going to be able to do a thorough job any time soon. He has added some biocide which should stop things getting worse, and reseated and sealed the filler cap mount – a probable route for water ingress. There is a wealth of opinion on how to deal with diesel bug. For every opinion I have found, the exact opposite was also expressed – use an emulsifier, use a de-emulsifier, don’t use either, keep the tank as full as possible, keep the tank as empty as possible, etc. etc.