Ive got a really weird problem
I pulled out a vertical and a horizontal guideline from the rulers. To centre them on the artboard so that the lines intersect in the middle of the artboard I set the align panel to "Horizontal align center" and Vertical align center" and checked "Align to artboard".
What then happens is the verical guide line disappears and the horizontal line stops at an anchor point on the center line of the artboard.
Any advice very welcome. O am obviously doing something wrong.