I'm creating 2 objects (a fence post and panel)
rec 0,0 and 11,10 for the post
rec 3,10 and 8,189 for the panel
which works fine
I then make 15 copies by using the move copy function
I select both objects and click on the botton right point as the reference.
Then I duplicate them at 11,189.
It does exactly what I want, produces 15 panels in row, except the position does not follow the grid the panels, they drift to the left. (on to someone elses property :-) )
What am I doing wrong or is this a bug, it should just follow the grid vertically?
Your procedure is correct, to be sure I have reproduced it and the result was o.k. It looks like you have accidentally engaged snap to an unintended point instead of strict command line use.
With snap it would be easiest to copy from "snap middle" of bottom line of post to "snap middle" of top line of panel.
I just tried a new document, one rectangle and no playing with snap. The result was perfect, so it must have been something I'd done with the snap settings, I'll try and be more careful in future and I should have tried that before posting.