Mist be a way to separate the two fuels DIY? Surely?
Two possibilities spring to mind, but neither is particularly straightforward...
Either 1. Gently heat the mixture and allow the petrol vapour to evaporate in to a home made condenser and collect the run off: should be neat petrol.
Or 2. Chill the mixture to approx -30 and simply pour the petrol off.
In each case you should be left loads of diesel for the boiler and free petrolly type stuff to use up in which ever pre '86 car takes your fancy

or donate to KW for the Westie

1. Is probably the best course, although extreme care should be used. If in doubt, don't heat it, but will take a lot longer to work. Thinking 30-40 degrees in a water bath, but obvoiusly try and keep the temp well below the flashpoint of petrol.
If you know any Science teachers, you might be able to blag the use of some Bunn flasks/glass tubes/rubber hoses and bungs and a Leibig condenser for the Easter hols. Just thing of it as you own little science project
