Hi Fader,
It's difficult to give you specific info without knowing exactly what the neck problem was or is. Was it an injury that's getting better or is it an on-going problem that's chronic?
I stand to be corrected but from what you've said it sounds to me as though the concern was regarding the range of movement that you have in your neck that is concerning the folks at the CAA. I suspect the flight test is to see how your range of movement translates to you safely piloting an aircraft.
Flight tests are conducted for all sorts of conditions and from what I know they don't want to fail you, they're keen to give you the chance to show you can fly safely within your physical capabilities. Please don't worry, see this as an opportunity to get your medical sorted and as a step towards completing your license.
It's annoying but it's one flight and if it has to be done, it's probably easier, quicker and cheaper in the long run to do it rather than fighting against it.
I hope it goes well. Hopefully you'll update us with the outcome.
Best wishes
B