I looked into this a couple of years ago..
1.One problem is when the engine is cold, because the veg oil is thicker than diesel it can damage your fuel pump.
2.Keep a log of how much you use

then if you get stopped you can show it and then you would have to do the paperwork and pay duty of about 24pence a litre on the amount you are declaring.....
3.I have seen some people say never use more than 50% veg oil to 50% diesel unless you have done a conversion.
4. Conversion kits heat up the oil, and you throw a switch when you are almost finished driving and it switches to diesel for your cold start the next day.
5. I did about 20 litres and it worked but my van did smell like a chip shop

I think the website was dieselveg.com
Hope that's useful.