In reply to Bjartur i Sumarhus:
> Heating oil and diesel are virtually the same thing, fill your oil tank at 97p a litre and syphon some out for the car.
The missing additives aren't hard to buy legally, either, if you care about the longevity of your engine.
> Expect most people on heating oil are doing this or will be in the near future.
I don't believe those cheerful excise people are very happy with this sort of thing, will be interesting to see if their roadside sampling starts to rise. I'm not sure if all the ways used to identify red diesel and heating oil are well known (beyond obvious things like the dies). It's a big risk to knowingly take - I won't even use a jerry can formerly used to move some heating oil for car fuel incase it introduces a traceable contaminant.