In answer to the square/round nozzle point. I think it's a great idea (even though it would put me out of business overnight) but just think it through a little.
Every pump and vehicle in the country would have to have the nozzle/filler neck changed as appropriate. That would be both hugely expensive and near on impossible
They have already changed the nozzles when Diesel became a popular consumer fuel... They mad the petrol one smaller 
Frankly, this situation aside as it is just pure stupidity (can they walk and talk at the same time?
)
, almost every case of misfuelling I attend is down to lack of attention or distraction. It's much easier to do than people think, just a momentary lapse in concentration and it's done. As an addition, some research done recently into the process of refuelling found that it is something that 99% of drivers do subconsciously, i.e. without thinking about it 
That is entirely my experience too. You could put warning stickers all over the instruments, door handles, filler flap, filler cap, in the back window and one on the roof just for good measure, yet people would
still put the wrong fuel in. It is simply because they are not paying attention to what they are doing. Most of the idiots that do this, and we see several a week on average, drive the same way.
We had one bloke working for us who managed to misfuel a recovery truck.
One of the window fitters where I used to work managed it 3 times in 2 months, until I trained him against it by charging him the full cost of the third time. By the time we had a recovery to base, another to the mechanic, his draining the fuel, getting it running again, a full tank of diesel to be sure and 2 days replacement vehicle hire it came to £700. When complained to the boss I went with him. On being told that it was the third time in 2 months, Con turned to him and asked if he wanted to pay for the other 2 times. He was a good man to work for!