Hello,
I have the same problem - I've not diagnosed the fault yet, but I've done a bit of reading; here's what the
workshop manual says for a 300tdi (mines a Td5, but I don't think the PS system is that different in terms of function):
Insufficient pressure in the system is usually caused by low fluid level or PAS pump drive belt slip, or one of the following: PAS system leaks, faulty PAS pump relief valve, fault in steering box valve and worm assembly, leak at piston in steering box, worn components in PAS pump or box.
In the past, I've found low fluid to give heavy steering, but I know that's not the problem this time with mine...
All the best