Pentax K10D & Sigma EF-530 DG ST PTTL Flash

I have just received a Sigma Flash Unit EF-530 DG ST PTTL from overseas for use with my Pentax K10D.
Problem is that while it goes OK on manual if I set it on "P" setting the lens stays wide open even though I'm using the new Pentax kit lens with aperture ring on "A".
Would be interested in reading if anyone else has this problem or suggestions.
My theory is that either the distance measured by the camera's AF is not getting thro' to the hot shoe or the hot shoe distance information is not getting thro' to the flash unit.
With thanks: Ian McHenry