Arne, you're a star! I assumed that Alt-Tab would be useless, as I couldn't see Bryce on the task bar, but when I tried, there it was.  I couldn't see it in Task Manager initially, but while I was messing about with Alt-Tab, the Bryce window just re-appeared! It seems to have been working happily in the background, as it's done another 5%.

Thanks for your advice - much appreciated. I can go to bed happy now!       8-)