Just spoke to Edgesports UK and they confirm that the F1 flat mount Juins are designed specifically to fit 160 rotors so no way to use with a 140 (at the rear anyway). They are pretty hard to get hold of so I'm gonna offer them for sale on the other disk brake thread before I return them.
Calipers are never rotor size specific. Just get a 140 rear theres plenty about and you wont notice a difference between that and 160