If you'd put more than half a tank of unleaded in, the engine would be misfiring like mad, if it ran at all. It certainly wouldn't go 225 miles on such a mix of petrol and diesel.

If you have receipts from filling up your new Golf, gather them together as evidence. However, you don't have to show them to the supplying dealer.

Remember, you have a 4-week old car that has broken down. It's under warranty. It's the dealer's problem, they need to fix it AND give you a courtesy car while it's being fixed.

If they try and blame it on misfuelling and seem like they are not going to proceed with a warranty repair, and you are 100% certain that you have never misfuelled the car, that's when you swing into action with your evidence.

they are correct, it can. However, if you are certain and can produce the receipts, then make it clear to the dealer in writing that this is not the case here and you expect a full repair under warranty. Also point out to them that you expect them to handle the old pump with care to ensure no petrol contamination can occur after removal from the car as you are aware that the pump supplier (Bosch?) will undoubtedly perform a chemical test on the pump to ascertain whether the car was misfueled.

Take it from me that suppliers of fuel systems specifically exclude misfueling from warranty terms and reserve the right to chemically test all returned components. And they do too.There is a significant number of misfuels being passed off as warranty problems out there.