I was playing about again with the iChat method of screen sharing without B2MM (Back to my mac) active in .Mac, and having trouble, because, I found out B2MM was active and the two modes don't work together. Off with B2mm in .Mac prefs, and up it came. :)
Then I discovered all this buggering about with scripts and ignoring error messages was unnecessary - Chax allows you not only to auto accept, but to only accept from specific people!
What a joy to sort it out.. it works in both modes now i.e. with B2MM on or off!