Over the past 4 months I have been suffering with neck pain, pins and neddles and numbness in my fingers on my left side. But I have also been losing muscle on the left side of my chest and in my left arm.
If anyone has any ideas about what the problem might be I would be grateful.

Sounds like something is pinched in your neck causing the pins and needles sensations. Have you seen a doctor about this?

We know nothing about your age, gender, history, and general health now, and you need a complete physical. So see a doctor. You could have any number of problems that range from mild to serious. It could be rare, it could be slow and degenerative, it could be something hard to diagnose at first, and there are several that are the 'rule out' type. It could be from a stroke you didn't know about, or just a pinched nerve. Or MS, MD, ALS, Parkinson's... anything.
And if MDs can't help, see a chiropractor.

Sorry. I am 37. Male.16.4 stone. Do regular exercise. And have played American football for a number of years. Not much history to tell but my father does suffer with Parkinsons. Have consulted my GP. Had physio and am awaiting an MRI scan at present. Did receive a big hit to the back of the head this season but x-rays showed no damage. Hope this solves some questions.