Run it neat if you want to, I do on my mondeo 1.8TD, tescos Grapeseed oil is 97p a litre, same as veg oil near enough. Just mix it 50/50 if you want to be cautious, in the summer is much better for it as well. Don't need to declare tax either, just keep all your reciepts, as you are allowed 2500 litres a year before you have to start paying extra tax.