Hi Frax,
I presume this is on a 300tdi engine? The easiest option is to put it in a high gear with the handbrake on, however this will only work up to a limit until the clutch slips. The other option is to remove the starter motor and lock the flywheel, some people just wedge a socket or something between the ring gear and the engine housing, but I preferred to quickly knocked up a tool which bolts on where the stater motor fits. I'll try to dig out a photo.
James