Hi Pat,
Yup you got it! So now my overly mathematical brain is punishing me. The origin is set in the plan view and therefore is a repositioning with respect to x & y coordinates - the xy plane if you like. So now I go to a front view and am dealing with x & z axis (xz plane) but the orthogonal repositioning of the origin seems to be relevant to this also. I had it in my head that somewhere there was an absolute origin represented by a cartesian origin of (0,0,0). I thought that changing the origin in the xy plane could not affect the z, but it seems that all I was really seeing was an offset on the left hand ruler. I assumed that when I went to front view the left hand (vertical) ruler was advising me about the relative position of objects with respect to the absolute origin of the layer in the z axis, but if fiddling with the origin in the xy plane then affects position in the xz plane then the scales are just rulers and the fact that I had a dodgy bottom z was in the end meaningless since it was just a ruler offset.
So I am left thinking that I should keep the origin at the centre of the drawing and this will mean that the OIP gives me good advice about where an object is with respect to my overall model space (made of many layers each having differing elevations).
Am I right?