Ok, I think we have the smoking gun in our case. Had a house call from the Importer and local Authorised Service Agent today to perform the new brake bleed. However, I made them pull the master cylinder to bits and inspect first. Some pretty serious corrosion on both the bore and the piston. I believe that this corrosion is the source of the 'air' in the system. Logically if you have you own onboard bubble factory, you can bleed it all you like, but it's going to gradually go soft again.
Now that we know that it is corrosion, where is the corrosion coming from?
The corrosion appears to be at the point of greatest contact between the piston and the bore - the outside edge where the pushrod is doing the propelling.
I now have another theory and that the issue is perhaps not a galvanic reaction between dissimilar materials - but the effect is similar. What if it's arcing between two electrical contacts?
On the other end of the cylinder you have a power source, namely the hydraulic brake light switch. If there were some current leakage on the +ve side of this, it might be trying to earth through the hydraulic fluid (which is conductive) and exit stage left through the nearest point of contact. This would seem to be the outside edge of the piston where there is perhaps side pressure form the pushrod and less support of the piston by the seals. The erosion seen is in fact the result of minute arcing and presumably the arcing creates a tiny stream of gas bubbles, which then makes the pedal go soft.
I need a big drink now... Brain hurts.