Ok so the final result was that the German 28mm ones fit perfectly (see pic below).
Fgute sent me some pics of his and indeed they are 1 inch diameter. Although his appears to be a 2 piece shaft with the constant velocity joint whereas mine does not have this. The service manual for the V6 depicts both tailshaft designs but does not stipulate a difference between the size of the uni joints between the two models.
So my conclusion is that although all Australian capris had 1 inch unis, somewhere in the world there were some capris with 28mm joints.
My two piece version with new unis:
Fgute's shaft with CV joint and 1 inch unis:
