Re: muscle infection?
I believe even in the UK it's possible to pay for private treatment, right? I would do that. I'd find the best infectious disease or orthopedic doctor who takes private patients and get an appointment.
What could be going on is that your body is fighting the infection somewhat successfully. That could explain some of the issues coming and going, that it jumps out and makes you sick, then your immune system fights it back down to wherever the reservoir is, then it breaks out again.
And yes, the infection probably is why your neck is spasmed. That or the pure stress at going through all this.