what is the best diesel additive

There are so many. But I don't really think you need them. There are some, if you only have low sulfur fuel available may help a bit but today's diesel has everything needed in it, including detergents..but just like gasoline, not all refineries turn out the same quality. Stick with well known brands..Shell, BP. Chevron, Mobil.

cheers buddy I have put paticulate filter cleaner in it as I only do short journeys thanx for you reply david

Now that's fine. A different scenario alltogether. And You're welcome!