From a BBC website
http://www.bbc.co.uk/bloom/actions/cookingoil.shtml""Is running a car on pure vegetable oil legal? Doesn't the government
levy tax on fuel?"
It's all perfectly above board, and there's no tax to pay unless you
use more than 2,500 litres a year (enough to take you about 20,000
miles). Hang on to your receipts in case you're ever asked to prove
you haven't exceeded the limit."
I used to do this around 7yrs ago, collected waste veg oil from local Kebab shop, filtered it though a bed sheet into a large bin that had a tap about 1/4 the way up (so remaining crud could settle below)
I ran a car on this neat with no added diesel for two years with no probs at all. Traveled all over the country.
Why did I stop? I found out that at the time it wasn't strictly legal and peeps were getting done for it. It seems the law may have changed now so maybe worth another look?