Thanks Jacob,
Right... If the new door had been the first door in the hole, I wouldn't have had an old one to compare with. So I didn't need the old door anyhow?
The SA door came as a standard, (Pre-metric) hence the reason I matched it to the old one. However, that still doesn't explain why the new door, being exactly the same size and shape, (Even if it was a trapezoid) was leaving a meeting gap that was twice the size of the gap for the old door. Which wasn't acceptable. (The hanging stile was okay). As I adopted the same procedure everywhere else in the house, without problem, then I can only assume I didn't match the doors properly, or the frame was out of plane and out of square.
Anyhow, like the chair with one short leg, I wasn't about to mess with it, so I re-hung the old door. And until I get the doorway moved, there it stays!