Over at TDI Club, one member from the San Francisco area bought a 2002 New Beetle TDI about six months ago, paying over $10,000. She has driven 7,000 miles using WVO in a two-tank system. Despite her best efforts, she now needs a new engine, at a cost of about $5,000.
Perhaps it wasn't the last 7k miles, but the miles put on by the previous owner. The fact remains, nearly EVERYONE who switches to WVO or SVO, has or will end up paying more for repairs than they saved on fuel.
Direct injection engines are, by design, not very suitable for conversion to run WVO or SVO. This remains true even for those using two-tank systems, where the veggie oil is heated before the driver switches over from diesel to veggie oil, and then switches back before shutdown, to rinse all the remaining VO from the system.
Unless I'm mistaken, some folks here are pouring SVO directly into their main fuel tank. This would be ill-advised for those with VE engines, but seems especially risky for those with PD engines.
I do not care how much fuel costs in the UK or anywhere else. It is not high enough to justify the damage that you will do to your engines by running WVO or SVO, no matter how careful you are.
Buy biodiesel, or make your own if you think you can do it properly. But at least recognize the difference, that biodiesel can be safe in TDI engines, SVO and WVO are never safe.