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I have an older diesel which I understand is not common rail, an 2002 Isuzu NPR motorhome. If I
inadvertantly put a small amount of petrol would that have any effect?

If it's not a common rail diesel and you only put a small amount of petrol in, that shouldn't cause a problem. Older diesels can usually tolerate a couple of litres or so (perhaps up to 5% of petrol). Make sure you fill the tank with diesel though to dilute the petrol as much as possible.