Well, I've had lots of helpful replies - thankyou - and have been thinking things through. The latest problem with my current machine - thread sticking in the tension discs - is obviously not my fault, and is one of several problems that keep recurring. I think it's time to admit defeat and go for a new machine, definitely from a different dealer. I don't actually think I'm as incompetent as he implies - I do pretty well on other machines - and on mine when it deigns to behave itself. I've emailed a couple of dealers that are some distance away but which I know have a good reputation, and one of them has left a very positive-sounding phone message, so I'm going to talk to them tomorrow. I'm probably looking at the Janome 6600 or 7700 - the latter has always been out of my price range, but at the moment some dealers have a good offer on it, so it's probably a "now or never" decision, as I understand that Janome's prices will be going up soon. The frustrating thing is that neither dealer currently has a 6600 in stock, and I really want to try both in the same session. I may also go and see the local Pfaff/Husqvarna dealer, just to see what they have to offer. The priority is a machine that's reliable for free motion quilting, and will take a range of threads, unlike my picky current machine: my vintage ones can handle the piecing.
I'm going to post another thread for comments on that.
Thanks again to all who responded: I really needed the encouragement and hugs when I started this thread.