I also am having power steering problems on my 1.9 TDI, no power assist at all. It occasionally works from cold, and once came in whilst driving. I have found that when the system works from cold, the warning light goes off after a few seconds with ignition on, but not starting the engine. This is apparently a system check by the ecu, and if the light stays on, it indicates a voltage problem (eg earthing). Yesterday, I cleaned the battery earth and the two earthing points near the pump (had to remove front bumper to gain access). Early days, but PAS has worked so far.....

I found a very useful website on the VW Polo forum which is a self-study course on the electric power steering. It explains in relatively simple terms how the system works. Hope you find it useful.

 

http://www.volkspage.net/technik/ssp/ssp/SSP_259.pdf

Looks good!