Hopefully i can help..
Miss fuelling a diesel engine with petrol can pretty much destroy/wear out the injection system,if doesn't normally happen instantly it can happen over set amount of short miles,even in some diesels about a couple mile down the road before giving up - but 120miles on wrong fuel doesn't normally happen on most diesels without some sort of power/or engine running problem surfacing first/lots of smoke out the back..Petrol acts a solvent so it basically washes out any lubrication that diesel has/leaves within the injection pump and the injectors for them to operate properly..if petrol in high quantanites is added to the tank then this whilst it runs through the system causes wear,lack of fuel pressure and eventually causes fine particles to run or damage the injectors..normally missfulling diesels is about upto 5 litres is about the max you can put in by mistake to be able to risk topping the tank right up with diesel.....however normally its best practice to actually drain the tank...considering you received the vehicle fully fuelled which they will have a note off during inspection before releasing the car or probably note of the previous user if they claimed back fuel expenses with receipt's/copied receipt's noting what fuel was put in...normally to prove whether it been miss fuelled or not they need to take a sample of the fuel and have it tested to prove other wise..but as it stands and from what you have said if you acquired the vehicle with a full tank of fuel then there would be no reason to fill it up/top up after only 120miles..
Thanks Robert. My opinion is that the previous renter probably just topped up the vehicle to avoid the hire company charging him the higher rate to fill it up themselves. With the recorded mileage and the amount of fuel still on board when recovered, both of which I can evidence then the only way I could of misfuelled the car would have been to remove the correct fuel and replace with incorrect! If he's put 5-10 litres or so in then do you think i could have made that journey without it stopping? It was pretty much all motorway.
Its hard to say really considering we or you dont know how much petrol had been in there.some diesels can go for a certain amount of miles some dont even get off the forecourt before cutting out.some get away with it by keeping the tank topped up to dilute the petrol.but this would only be advised if the ratio of diesel was higher over petrol so if you put in say £3 -£5 by mistake into an already tank with diesel in it - then you just brim the tank with diesel and it should be fine...if its way over that like £10 and above then you wouldnt be advised to drive or even start the engine but instead have the tank drained out aswell as the fuel filter replaced if the engine was started..continue to drive with too much petrol over diesel - then the end result is as i explained before..But again depends on the "amount" of petrol was in it..but you would always get some sort of warning from the engine itself if that was the case..
your probably right that maybe at some point before the previous renter or even the hire company before re-rental had put the wrong fuel in,they would have to look back any records they have from previous rentals and any receipt's/copied receipt's they have...because there wouldnt be any reason for anyone to be truely honest with a full tank doing only 120miles would consider filling the tank back up..unless there was something seriously wrong with the car overfuelling or the fuel tank,fuel system had some sort of leak to merit having to fill it...