I had a VW Passat diesel on demo several years ago. After a run the engine wouldn't restart very well, it took ages and nearly ran the battery down.

I bought a Passat in the end and asked what had been wrong with the demo car. They said someone had put petrol in by mistake, they also asked if I would like to buy the demonstator car. I said no!

So yes mistakes can and do happen on demonstrator cars. If you really have not put any petrol in by mistake, however little, keep on to the dealer to sort it out.

I do not use supermarket fuel, I use Shell Nitro in my 5 series. But supermarket fuel is perfectly OK to use.

The dealer cannot invalidate the warranty unless they can prove that you misfuelled it. You would know if you had ever misfuelled it as the engine would run very roughly and probably stall / refuse to run altogether.

If you have genuinely never misfuelled it (I don't mean this to sound accusatory) then one of two things has occurred.

1) The pump has failed under normal working conditions (which can happen), which means it's a warranty job and should be repaired at zero cost to you.

2) The car was misfuelled as a demonstrator, in which case it's the supplying dealer's problem, not yours.

Did you buy it from the dealer that is now investigating the fault? If so, and they continue down the "you misfuelled it" road, then stick to your guns, assert you have never used the wrong fuel, and counter-argue by asking how they can be certain it was never misfuelled during its time as a demo car.

Don't worry about using supermarket diesel. All fuel sold in the UK has to conform to minimum standards which meets Mercedes' requirements.