I brought this occurrence to the attention of Cheshire East Council Trading Standards, though I don't know whether I expected a reply.
What I'd forgotten is that Cheshire East Council makes regular appearances in Private Eye's 'Rotten Boroughs' feature.
In the magazine's first issue of this year (No. 1461), Cheshire East was declared to have won the Filthy Liars of the Year Award for 2017:
'Cheshire East council was caught out having falsified air quality figures in a "deliberate and systematic" manner over at least three years. By pretending air quality in certain areas was far better than it actually was, the council was able to grant planning permission to housing developments which should have been vetoed on the grounds that the extra traffic generated would cause unacceptable levels of pollution.'
Perhaps the petrol station manager thought that by diluting fuel with water he was helping Cheshire East to reduce pollution.
More seriously - and perhaps relevantly - the 'Rotten Boroughs' page in Private Eye No. 1462 reported that 'With bullying allegedly rife [at Cheshire East Council], the Local Government Association is called in to carry out an independent inquiry into the unhappy ship.' The magazine gave as an example the story of a whistle-blowing council employee who was harrassed to the brink of suicide by Cheshire East executives. Which department did he work for? Trading Standards. Hmmm. No wonder a petrol station suspected of malpractice went uninvestigated.