Hello All,
To close the loop on this I finally managed to get back to fixing this problem. I didn't touch the stuffing box at all. I took the information Christian Williams provided and fitted a Zerk fitting to the rear side of the fiberglass rudder post about 10 cm down from the top and pumped it full of marine grease (14 oz) until the grease started to come out around the top of the post. During the process I rotated the rudder back and forth to encourage the grease to move completely around the post within the tube.
The leak was completely sealed. I have since tested the boat in very rough and calm conditions, under power and or under sail and there are no leaks at all coming from the rudder post.
Note The installation of the Zerk fitting was very quick: just drill the hole, epoxy the Zerk fitting in with good epoxy glue and some glass mat and wait overnight for it to harden, and then pump the grease in.
Note I had fairly easy access to the back of the rudder post from the rear locker so there wasn't too much body contortion required and much less than if I had tried to get at the stuffing box. I would recommend this method as a good way to start if anyone else has a leaking rudder post. The other upside is that the steering now feels very smooth because it is well lubricated again.
Thanks Geoff