Now that enough time has passed, and my embarrassment has somewhat subsided, I can own up. I did it!
Fortunately it was on a nice warm day during the summer.
My only excuse is that I was definitely set up.
Foolishly filling up at a large supermarket, which has all the pumps close together, I lifted the nozzle out of the DIESEL holder, and pumped away, thinking about the work I was about to undertake. Unfortunately, the tank only had a couple of gallons in it, so I put about 50 ltrs of petrol in. If all the nozzles had been correctly positioned in their holders, it would not have been possible to operate the nozzle I had in my tank.
After I had sorted my problem a mile or so later - siphoning the tank into a collection of plastic cans, topping up with a couple of gallons of diesel, removing the filter and refilling with diesel, - I went back to T****'s to see how I could have made what, as Kevin says, is a stupid mistake. What had been done was certainly deliberate, the diesel and petrol nozzles had clearly been swapped in their adjacent holders, and the diesel nozzle just balanced such that the petrol holder it was 'in' would still pump. Stupid design, which allowed a stupid mistake.
Yes I know, I should have noticed the green decal on the nozzle, but as said I was miles away as I pumped, and quite happy in the knowledge that it came from the correct holder.
You have been warned, there are some a@?&hol;s around, so check not only the pump, but the nozzle in it.
On the plus side, it happened to me, and I am quite capable of sorting it out, and not some hapless individual driving an expensive car, which would have cost a fortune to fix. And SWMBO's Jazz had lots of extra - if slightly oily - petrol to use.
I was concerned that I may have damaged the injector pump bearings, but after 3000 miles, all seems well.