Yeah that's the thing, really depends on the day as regards cetane.
The specs are minimum specs, so you could get a batch from one refinery with the bare minimum amount of cetane which as stated could go to a supermarket forecourt or to its own branded forecourt or a competitors forecourt.
The following delivery may come from a separate blend with a higher cetane content, no way of telling really.
The thing is vehicle manufacturers state a minimum spec of diesel for their engines to perform correctly on which is what will be supplied at the pumps.
Sure by all means boost your cetane by Rhino or Millers whatever that's up to you, or just pay more and buy a premium diesel that has a higher cetane value already in it.
Personally I stick to the standard stuff and stick a bit of diesel redex in every other tankful never had any running problems.
What is important is a regular change of the fuel filter on a PD engine as they get crapped up really quickly.
I changed mine after 8000 miles and 2 years, decided to cut it open and see just how dirty it was. Trust me I wouldn't want to run past 10K miles without changing one.
It's a 10 min job and they self prime.