NavList:

Hi Dave,

Thanks for your response.Um, I suppose I was being naive in assuming that "chart" Lat and Long would be based on a notional sphere where meridian was defined by one's preferred datum and Latitude by the angle between the equator, the centre of the earth and the observer, measured perpendicular to the equator. 

I can understand the need of surveyors to be concerned with the distortions in the shape and variations in local gravity of the earth and, presumably the effects on sattelite orbits and, hence, on GPS output but this shouldn't affect the results of celestial fixes.  

Have Just received Frank Reed's post - clearly it's back to the books to get a better understanding of Deflection of the Vertical and its significance.

Tim I.