MumsyT ... First of all, I can't answer your question directly about the 570.
About the 790 that you are considering ... if this dealer cannot figure out what is wrong with the 570, are they going to be any different if/when you should have problems your new 790?
When buying a machine, one really needs to consider the after-sales support and service. Things happen to our machines, and If they can't service your old machine, is it realistic to think things will be any better with a different model of Bernina?