Yikes - just got around to checking the progress.
Have currently done 862/9429... it is increasing, so it hasn't completely stalled like I first thought.
Been 68 hours so far. At the current rate I calculate it will take about a month to complete!
Looks like I'm restoring from backup. Is there likely to be a revised upgrade procedure released or should I wipe the computers table (as suggested by wierzbowski) and try again? Like wierzbowski, I'm more concerned about preserving custom reports etc rather than computer history.