[AaronLancs]

Wasn’t the Six meant to be British stories of the day, whilst the ten was the same but with more international affairs?

Sorry to return the discourse to this but I wouldn't say the Nine / Ten was exclusively 'international' per se but one would suspect over time it had become the television equivalent of Radio 4's The World Tonight. Becoming the "main flagship" bulletin that took advantage of the 'digest' sort of format for a news bulletin where is you wanted a more rounded bulletin it was the appointment to view, BBC One 9pm or 10pm Monday through Friday.

Incidentally, one thing that didn't help was in 1999 the location tags in the titles being the same for the 1 O'Clock News & 6 O'Clock News:

BBC 1 O'CLOCK NEWS TITLES 1999
tvark.org 

BBC 6 O'CLOCK NEWS TITLES 1999
tvark.org 

And the more internationally 'flavoured' title were for the 9 / 10 O'Clock News and the main generic bulletin titles:

BBC 9 O'CLOCK NEWS TITLES 1999
tvark.org 

BBC 10 OCLOCK NEWS TITLES 2000
tvark.org 

GENERIC TITLES 1999
tvark.org 

Hopes this adds my two penny worth of looking into this.