Right, for those veggie virgins out there, a couple of points to be aware of before you leap right in.
Firstly, as makeitfit said, no modifications are needed on a 2.7 T2 for it to run, and run well, on straight veg oil from the slagmarkets, the 3.0 will NOT run well and WILL damage the engine as it's a very different beast.
Veg oil is more viscous than diesel by quite a bit at any given temperature so it will add extra stress to the pump, the 2.7 TD and TDi's are either fitted with a Bosche or Zexel copy of the Bosche pump that has been well abused by other 'veg burners' in other motors and it has a good reputation for taking the strain without much issue.
That said, in sunny Scotland, as with many things, veg is thicker cos it's colder up here. I've been told by others that have done far more study into this than me, that roughly 20% petrol added to veg oil will bring it into near diesel viscosity depending on the type of oil used. Rapeseed (asda/tesco 'veg oil' is usually rapeseed if you look carefully at the packaging) is the best, followed by sunflower. Soya tends to be thicker and what is cheapest in Booker, et al. Best test is grab a few jam jars and mix up a variety of petrol/veg cocktails (VERY carefully) outside and then leave these out over night, those with less petrol in will go cloudy faster.
Diesel does thin the oil too, but diesel is already thicker than petrol and it freezes quicker than petrol. It's also dearer!
Burning veg also has it's risks to the engine, first one is that the crud in the tank from years of diesel will possibly dissolve in the veg oil and clog up the fuel filter and the banjo filter is a metal gauze which can block with veg fats too.
Your pump is under more strain with veg unless it's well diluted so there is a risk of long term pump damage, for me, considering the savings made, it's worth this risk.
If you only do lots of short journeys on neat veg then there is a small risk of unburnt veg oil building up on the rings, known as ring gumming. If this happens, there is a risk of veg oil blowing past the rings and mixing with the engine oil. Engine oil and veg oil make a jelly that looks like really nasty Vaseline and will not lube the engine at all. Regular veg burners tend to change there engine oil a bit more frequently to minimise this risk, sometimes as often as every 3000 miles.
You MUST keep a log of how much veg you have put in the tank as you are only allowed to use 2,500L in any one rolling 12 month period without paying road fuel duty. HMRC have the right to demand this proof.
Nae sayers will blame any failure on the use of veg in your car as well; "My prop shaft failed cos I was running veg" or "Your CV joints have worn faster due to veg oil" or the likes. It's amazing what some people will try and blame on it!
If you are really worried, either learn to safely make bio (Do NOT buy a bio plant from the web without checking a decent forum on it's safety, there are loads of dangerous plants getting sold out there) or buy bio diesel in bulk to get a good price. Pump bio diesel is quite pricey these days as it gets fuel duty and VAT on it too, but still usually cheaper than normal diesel. And for those that say they've never used bio diesel, they are wrong as ALL pump diesel must contain at least 2.5% by law these days with some having up to 7%.
There, I think I've covered the worst of it, I don't doubt others will add to this.
Mine's a 90 litre tank, guess what I run my truck on :sly
