I looked into veg oil a while back and legally if you use it as a fuel you need to declare it to the taxman, and they will charge you fuel duty on it. So veg oil costs like 50p a litre and the duty is about a quid or something, so it ends up the same price as diesel. Also, you need to change the fuel filter more often as it gets caked up with stuff and the fuel lines. One of the biggest problem is cooking oil is thicker than diesel, so you can either mix 50/50 in the tank, or ideally use an inline heater on the fuel line. Some fuel pumps also dont like it, so you need to check the pump you have is okay as if you break the fuel pump then its not cheap!!!!
So, all in all, unless you are doing mega miles you arnt going to save much money - unless you dont declare it to the tax man, or unless you brew your own biodiesel using free waste cooking oil from your local chip shop.
One of the biggest negative points I saw was that your car smells like a burger van if your running it on cooking oil, so you end up day dreaming about burgers as you drive along, resulting in you spending a small fortune at Macdonalds each day which you wouldnt have done had you been running on diesel. So fuel savings will be negated by the extra expenditure on burgers!
Best old cars for running chip fat tho are old mercs and volvos apparently.