I'd like to add a post-script to everything I said here.
It's been more than 4 weeks since I went for an investigative X-ray that would, hopefully, reveal why I'm losing mobility so quickly. We're not talking "I can't be bothered" to walk, but that I actually can't at all. Since that X-ray, I've found myself stuck in the bath, I've required the assistance of 2 people to get to my feet, and I'm now crying out in pain every single time I stand up.
Yesterday I called my doctor, again to beg for some kind of concern and action, and she was surprised to learn that I've already been for an X-Ray because she can find no mention of it in my records.
This would be because no-one has bothered to put it under the nose of a consultant for examination and report yet.
The NHS may be wonderful for cancer and heart disease (and I'm truly delighted that it is - congrats to all survivors <3) but it's far less concerned about anything that isn't immediately life-threatening.