I have a friend who makes Bio Diesel machines he runs 100% Bio in the summer and 50% Bio & 50 % pump diesel in the cold weather, he seems to be regularly changing his fuel filters something like every four months or so.
If you use new vegetable oil straight from the can or the supermarket shelf tax is due to customs and exercise. One of my friends customers was using 50% new vegetable oil and 50% Bio, C & E fined him and I presume had his van off him as well just to top if his good day.
Tax wise you can make 2500 litres for you're own personnel use but you must keep records. Don't be tempted to sell it to friends if you make a batch thats poor quality only you suffer make it for a friend it buggers up his engine you're liable and if C&E finds out you're liable for a nice fine plus the missing duty.
Making Bio Diesel involves the use of water, methanol and caustic soda at its most basic level.
If you're making Bio yourself you end up having used vegetable oil stacked up around the back yard / garage. Not the most pleasant smell plus you have the waste products to remove as well. The glycerol caustic soda mix can be made into soap and I believe a handy brick for the barbecue if mixed with sawdust.
Vegetable oil isn't all the same liquid either some vegetable oils have different points at which they will be either liquid or set to a jelly like consistency and be unusable in the making of Bio Diesel. Knowing which oil is useable and which isn't takes experience and collecting from fast food outlets that don't tend to cook burgers and the like in the oil, animal fats can not be broken down in the process to make bio diesel.
Thankfully I have no where to store used oil I might have been tempted to do it myself.
Lastly the collection of waste oil requires that you have a waste oil collection licence at £150 for three years.
I'm know expert on Bio Diesel I don't make it myself but I do tend to listen and take on board the reasons why I don't fancy trying it myself. I would have been really keen if the Ferret could use it really really keen