I'd check out some of the forums that specialize in bio fuels. I'm more into LPG but know a few people that run Bio. It also depends on your vehicle as some are better than others and can run a higher Bio %. You right about getting oil to process yourself.
However, with the prices as high as they are at the moment even a trip to your local Indian wholesalers is worth a trip. They tend to sell 10-20L veg oil and the prices are certainly a lot less than the pump prices.
Some people have reverted back to using heating oil as it's the same thing but around 67p per liter I think depending on the supplier. But, that is illegal and you will get fined if caught, I hear the fine is £500 so think about it

But before you get into it read up on it and check your van is suitable first, also check if your van is on lease or under warranty still as Bio might invalidate it.