Hi Mike,
I am having the same problem at the mo!
See this thread:
http://928uk.myfastforum.org/ftopic84.php
I have discovered that you can clear the fault by disconnecting the battery but it appeared again 3 journeys later!
I am going to change the pads and bleed the system then see what happens. After this I will check the fuses/relays/electrics etc but I don't want to rule out that it could actually be brake circuit failure.
My car is an 83 S Euro spec.
Best wishes,
Llew