You should be able to go higher than 5% - I would expect 20% without issues, and quite possibly you could go higher, though I have no experience of your engine.
I run a 2001 Rover 75 CDT on 100% biodiesel (home made) and have done for the past 18 months. 85% in the winter. No issues, runs perfectly, better than on petro - nice and quiet purr.
My wife's Skoda Fabia 1.9 SDi also runs 100% - both happy.
Be sure the biodiesel you buy is of good quality, properly washed and dried.
If running 20% and above, you may benefit from renewing the fuel filter after the 1st 500 miles on biodiesel, then again at 1000 miles, because it can loosen up some of the crud which will have been deposited by the petro diesel.