Have considerable experience of "taking on" a large multi national oil company,who denied that the diesel they supplied contained any impurities.
Dispute involved over 100 vehicles,and took over 2 years to resolve.
We had to engage a university professor,an expert in Marine Chemistry. He was able to prove that the diesel contained salt water.
This was a very substantial claim,and I think you will find the oil magnates will continue to deny any liability. It would be better if you all clubbed together to make your claim thus reducing your considerable potential costs.
Always bear in mind that these oil moguls have been known to bring down governments.
I wish you every success in this very difficult matter.