[Scribus] PDF/X-3 Option 'Disabled'

[scribus at wittongilbert.free-online.co.uk]

>>When I choose Export to PDF I get a window with a series of Tabs along
>> the top.
>>Can anyone explain how I enable the PDF/X-3 tab as it currently
appears
>> 'greyed' out.
>
>you propably have to turn on transparency capabilities in the
settings...
>
>regards, bostjan

?? Perhaps, but:

1, The documentation says turn 1.4 Transparency OFF
2, PDF/X-3 is supposed to be based on 1.3 and 1.3 doesn't support
Transparency - does it?

Tried Settings/Preferences --> Display 
With both 1.4 Transparency ON and OFF and no change?

Calum