Glojo,
according to UNITE the dispute is about the refusal to agree that there should be national standards on H&S for drivers, and a national forum established to oversee the industry to ensure standards are maintained.
The link below is to a page with a 4 page PDF described as a Q&A on the subject, which outlines Unite's taker on the matter, which should go some way to answer your question.
(
Enough is Enough - tanker drivers campaign )
Personally I am inclined to believe Unite on this rather than the PM, as a teacher (these days) I am more than aware of the amount of barefaced lying that went on when my career was being used as a dartboard.
The drivers appear to be complaining that the private sector has become too concerned with short term contracts at the expense of ensuring a sustainable oil supply system, and I don't find it overly hard to imagine there might be an element of truth in that.
Dave