I’ve had a response from Janome UK to my query over what my dealer said about not using machine quilting threads:
"You should be able to use any thread on virtually any machine adjusting the thread tension accordingly if required although you may get different results with various thread".
Interesting – last time I went to them to complain about this problem on my machine, I was recommended to stick with fine thread, and they sent me some embroidery thread as samples of what would be best to use.
I've made contact with another dealer, about 50 miles away, and am going to see him on Monday, to try the Horizon, and he's also going to take a look at my lemon, aka 4800.