I had a remarkable first session with a local craniosacral therapist, trained in the originial structural Upledger approach (as opposed to the Biodynamic approach). He has worked a lot with PD and says that while 'of course there is no cure' his patients' neurologists comment that they're not progressing as expected.
He said my neck was incredibly tight, and that nerve and blood flow to the brain was restricted. Amongst other things, he did something called a 'neck unwind' and I am now able to turn my head to the left and right without great pain (makes parking much easier!). I felt incredibly relaxed afterwards and my early morning tremors are greatly reduced. I am very early PD (undiagnosed) and he thinks many of my symptoms will have been coming from this.
I know many people on this forum feel they've tried everything, but I really would encourage looking into this. He is elderly (sadly retiring soon) and a stickler for the original Upledger training and less enamoured with the Biodynamic version of craniosacral therapy which is popular now (which once helped me with sitting bone pain before a flight home from Bali, in a way that I found near-miraculous at the time). Biodynamic people don't do the same structural work; they work via deep rhythms and energy.
Here is a case report by an Upledger trained craniosacral therapist on a PD patient to give you an idea of the kind of detail in their approach and what they can do.
Personally I would travel to see someone who could work this way (people travel a long way for him, but he rarely takes people on these days - however he has mentored two people in Sussex who he refers to, for any UK people interested).