I seem to have solved the problem. I remembered that the last time I had the TiVo working, that I had set the Swap file to be 256Mb rather than the default 127Mb. So I re-imaged with 256Mb swap file last night and GS without a hitch the first time through.

Now attached a sticker to the TiVo with Swap file size, source details, postcode setting, route.tivo info, and dbsetx commands - for getting it up and running the next time I break it