OK I put pen to paper (PC application actually) & drew the swing arms/ chain movement and positions for normal & loaded positions (6deg compression).
What I actually saw was that when the whell is pushed up, the front to rear sprocket distance is actually marginally reduced!!
So this means that contrary to what we say, there should be an increase in chain slack??
Unfortunately I can't see a way of uploading the drawing to find out where I've gone wrong. Shame!

I called my dealer & talked about this, they said they tend to set the slack to somewhere between 30~35mm when they do the PDI. So this is what I'm gone a do