I agree with your view, I have done a bit more riding and still notice the clutch takes up sooner when starting out, then takes up later once I have gone 10-15 kms. I will fit the original master cylinder piston assembly next time I have it apart. Thanks!
Several months later the clutch is working normally, I suspect the friction surface may have had some oil contamination from a leaking seal, since replaced.Cheers for all the good advice, a kiwi bike is never stranded for long!