Diesel in water Trident tank...

Hi guys, hope you're well. A quick introduction, my name's Ken and will soon inherit my Dad's T5 Trident. However, my brother has inadvertently put about £10 worth of diesel into the fresh water tank thinking it was the fuel tank (he is in no way familiar with campervans, so an easy mistake). 

This was done about a month ago, and the diesel has been sat in the tank since as we've been busy arranging funerals etc, but I was wondering if there is any way to go about draining the fuel and/or cleaning the tank? My worry is that it might require an entirely new fresh water tank as it has been in there a while now, but if that what is needed then we'll have to do that. I'd very much appreciate your opinion on the matter.

Thank you for your time.

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Thank you so much, exactly what we were looking for.

You will stand a better chance of success if the water pump has not been run at all. Once diesel gets into the pipework, things seem to get very iffy. Good luck and it would be very nice if you could give us some feedback as to how things go.