OCTYL NITRATE cetane enhacer, such as AMSOIL may be difficult to find in europe, at least in pure form. Most brands sell it extremely diluted with kerosene, so its real price skyrockets. Here an useful way to have it by post in pure form and neat price.
Go to http://www.vegetableoildiesel.co.uk/products.html and you will find this

-Veggi-Boost combustion enhancer is available in 500ml cans for people to create their own specialty fuel. It is essential to anybody using vegetable oil as a diesel fuel and will improve performance and help protect both the engine and the environment. Veggi-boost is a cetane improver and is added at about 0.1 % volume. Just a tiny amount has a large effect. It contains nearly 100% 2-ethyl hexyl nitrate which aids detonation inside the engine combustion chamber, enabling vegetable oil to be used more readily in a diesel engine. Also great if you're thinning veg oil down with petrol - it will restore the cetane value of the blend-

CHEVRON says.
Cetane Number Improvers (Diesel Ignition Improvers) Cetane number improvers can reduce combustion noise and smoke. The magnitude of the benefit varies among engine designs and operating modes, ranging from no effect to readily perceptible improvement
2-Ethylhexyl nitrate (EHN) is the most widely used cetane number improver. It is sometimes also called octyl nitrate. EHN is thermally unstable and decomposes rapidly at the high temperatures in the combustion chamber. The products of decomposition help initiate fuel combustion and, thus, shorten the ignition delay period from that of the fuel without the additive.
The increase in cetane number from a given concentration of EHN varies from one fuel to another. It is greater for a fuel whose natural cetane number is already relatively high. The incremental increase gets smaller as more EHN is added, so there is little benefit to exceeding a certain concentration. EHN typically is used in the concentration range of 0.05% mass to 0.4% mass and may yield a 3 to 8 cetane number benefit