I guess the issue now is whether the caliper can be freed off or if it needs to be replaced.
The car has 5yr warranty coverage so I'm a bit stuck with letting the dealer do the rear discs and pads and seeing what happens.
I have suggested to the service manager that their first look at the car, not hearing the noise, and declaring it to be OK, is not the level of expertise I would expect, and that I also wouldn't have expected to be replacing rear pads and discs at this point, but at the moment he's having none of it.
He's blaming my daughter as she baulked at paying the £140 they asked for to do further diagnostic work after they said they thought the car was fine and couldn't hear the noise. So she went away happy - she only took the car back as I insisted.