Having done a bit more research into the above, It appears that Biodiesel does not vapourise as well as conventional diesel, this has a double effect-because it does'nt burn as well as normal diesel it is more likely to end up in the sump and also not burning as well means that we have much lower exhaust gas temperatures and therefore much poorer regeneration.
So another part of diagnosis should be--where do you get your diesel from.

Ed Kaps