[Humphrey Hacker]

What I remember was that the data service offered came as multiple of 64Kbits/s
even 64Kbit/s was far more tthan any customer seemed to need -
so Im not certain that they had any .......
But I know that BBC datacast (and presumably DBI) had enquiries for their services at lets say 9k6bit/s or less
as a result of BSB advertising ....

That got me to look at the D2MAC spec I know it was D-MAC but the basic structure was the same
www.etsi.org 
Section 4 describes what it can carry as Data
4A data is Teletext in field blanking of the video
4B data is Teletext carried in the MAC mux
4C data is the General Pupose Data carried in the MAC Mux (this is the 64K service)

It would seem to me that 4A would take a channels teletext both Addtsional (Ceefax like)
and Ancilliary (This channel listings / info or Subtitles)
4B would be a BSB Text service which could be both Ceefax Like or data broadcasting
But Im not certain how much of this was actually used....

It's a shame BSB didn't stay around long enough to develop its technology .