I remember someone at work putting a similar cock up into buckets/tubs and then when the diesel settles on top of the petrol he just syphoned the diesel off the top, leaving the petrol in the tubs.
Thanks guys. What sort of quantities should I put in?? Couple of litres to every full tank of diesel?? Not brave enough to use cooking oil. I know loads of people do, but personally I always feel like it will naff up my engine so I have stuck with regular diesel. I dont do a lot of miles anyway so the benefits would not be great.