I have this problem on my TD5. I bought some Lucas power steering sealer, which is really thick oil and added it in. I bought another bottle when it had leaked enough to add some more. It definitely slowed the leak down initially and improved the feel of the steering a little.
After reading your post I had a look and was suprised to see that my steering box wasn't leaking any more. I had been keeping an eye on the resevoir and it didn't seem to be dropping too much.
I have been thinking about why it might be leaking from here, as the steering on my TD5 seems very light with little feel when cornering. I was wondering if the PAS system was generating too much pressure. Although, as the leak has stopped this would seem to contradict this theory, unless the Lucas is doing a really amazing job.
I can hold the car in a straight line easy and cannot detect much play in the steering system. If I run the tyres ay the 28 psi recommended it is particulary bad, but if I drop the pressure to 23 psi the steering is great.
After all that, your're probably going to tell me your steering is spot on
The RAVE CD has a diagram of the steering box which shows the plate.
I may also be able to dig out a thread on another forum on replacing the plate. I'll have a look tomorrow and see if I can find it.