Parked up in a layby ,when a daf rigid pulls in, ADR orange plate on the front dislpayed,nothing on the rear of the truck.
So i ask the driver where did you do your ADR course, as mine is due to expire shortly,to which he replies “whats that” . 
He had 2000 litres of red diesel in a yellow steel box on…and i,m thinking is it worth renewing my ADR 
Yes, it is worth renewing your ADR, but having said that it depends how much ADR work you are doing. If it is only now and again then it is up to you. But the person you mentioned was breaking the law (as far as I can tell) having done a little bit of research, the only thing I really came across was that you can only transport up to 1000litres of fuel without ADR therefore if the driver of said vehicle was carrying 2000ltrs and didnt even know what ADR was then 1) he has failed as a driver (know your loads and regs about your load) 2) TM should know if there drivers are ADR qualified for loads as they also have a duty to ensure that they and there drivers stay legal, in terms of who gets what loads!
I may be wrong, or people may disagree with some of the above but that is just the way I see it!! I know that I do not hold an ADR qual therefore I would not take an ADR load and would inform the office of that!! Unless of course the “ADR” load was under the qualification threshold!! Which is on the Gov.uk website.
The orange Adr plates let the fire brigade know there are dangerous goods, in case the driver was involved in an accident, and unconscious, they search the cab for the relevant paperwork then decide which way to fight a fire or deal with containment of hazardous goods .
Some nasty stuff out there can react badly to water or foam.
Diesel Dave may advise about this guy with fuel on board.
Having one plate up will attract the attention of Dvsa and the police, and maybe a fine.
You are not required to know what is ADR and what isn’t & how much you can carry (afaik) as it varies for each UN number, transport category and packing group. You would probably question it though if they brought diesel out. The transport office should tell you whether it’s ADR or not & the notes to go with it should include all the details about it such as class, un number, name and a few other things.