I don't see it incorrectly when I print or Print Preview. It shows the "minus" weeks.

Is your copy of Project 2013 fully updated with the latest public updates and service packs?

Check: http://blogs.technet.com/b/projectsupport/p/msp13.aspx

Its on screen thats incorrect?

Look at the two screen shots.

The first one is the PDF version which is correct.
Week 1 starts on the Project start date. Task prior to week 1 are shows as minus weeks.

The second image is a view from within Project.
Week 1 on screen is when the first task begins (prior to actual Project Start date). This is not correct.

What I am trying to resolve is the discrepancy.
I understand your workaround, but what I do not understand

A) Why the print is different from whats on screen in the Gannt chart?
B) Why on earth did they change it cos it works fine in Project 2007?

A is the priority - both the on screen Gantt charrt and the print(and print preview) should show the same thing
Whether its minus weeks or not is not important. They should match..

And if it requires an update that a pretyy fundamental slip up to have made inthe fiurst place.
I will check versions though.