Hi Andrew, welcome on-board.
A friend of mine wanted to convert his old diesel to veggie oil, but didn't found someone accepting to reserve him their used oil, so he bought a new low cost car.
Have you got problems to find used oil ?
Have you seen a lot of power loss between diesel and veggie oil ?
How much present is the odor of burnt oil ? I saw a veggie oil car once and I had the feeling it was cooking French fries
To use less diesel, you should build a front grill block, which will permit to heat faster. There is a lot of carboard/coroplast examples on the board.
As your 240D is a manual, you should try to use coasting. Associated to pumped up tires it should give you good results.
Have fun,
Denis.