When we got home with the van, it was after the place had shut, so not able to speak to them yet.
Also after dark, so can't see properly.
Also, no intention of doing anything which might release petrol fumes to form an explosive mixture.

My background is in biology and a big chunk of working in chemical labs, so am well aware of toxicity of petrol.

Initial thoughts after somehow draining the tank (Will look for drain valves or taps in daylight).
1) Petrol is volatile, so if tank is drained, warm air piped in will evaporate both petrol and water.
If completely evaporated, there should be no residue to produce a flavour.
But if tank cleaning successful, there may be still a mixture in pipes.
2) Alcohol - a solvent which mixes with both petrol and water. Used to be recommended for removing water from petrol tanks at start of winter. In theory should also dissolve petrol in a water pipe.
If petrol reduced to level below toxic dose, a flavour may be annoying but not harmful.
3) Possibly need to replace pipes if alcohol as solvent not effective.

Waiting to see if any mudcatters have direct experience of same problem.
Quack!
Geoff.