Not fully there yet but an update.
Have now proven conculsively that the problem was . . . . my phone line. Crackle in background led me to call my telco's line company to fix the line. Amazingly they came that same afternoon and found a "massive" fault which produced a very clear line when fixed. Both my TiVos have now been through GS without a hitch; the new one had a change to the parameters based on private info from a reader of this thread, but my old one had no changes from before. Both connected instantly when they got to the point where they previously failed.
What's interesting about this is that even before the line was fixed, I was having NO problem connecting via browser to the file sites and I could download slices using HTTP (or was it FTP) with no problems and at my normal speeds (about 1.3mbps). This suggests that the TiVo is far more dependent on clear telephone lines than a PC, something others may need to be aware of.
So now I get live TV via the TiVos and can make manual recordings.
I am still having a problem choosing the right parameters to get actual usable guide data and also can't get TiVo to change channels on my Topfield decoder. I've had advice from someone who also has Freeview satellite, but their parameters didn't result in success on the new TiVo, and I'm in the "4-8 hour reorganisation" part of the process on my old TiVo where I chose different (and I think more likely) parameters. Will report again once I've done further testing. I note that "Topfield" on it's own doesn't seem to be an option in settop box choice (it does have "OzTiVo Topfield" in the list of options but that didn't work at the channel changing point on my old machine and "Pace" didn't work on the new one). I'm sure lots of Aussies have Topfield decoders (mine's the TF4000Fi) so any advice on the option I should be choosing here would be helpful.
David