Sleep apnoea

I was diagnosed with OSA about a decade ago. I didn't go down the recommended CPAP machine road and left it untreated. Now I need to get it treated.

What are the steps I need to follow? i.e. Do I get another sleep study?

Do I need a GP referral for a sleep study?

Who is best at carrying out quality sleep studies?

My doctor doesn't care how much all this costs. Can I get help via a cheaper route?

How do I get help that isn't geared to me purchasing a machine if not necessary?

I am a pensioner and also have end-stage kidney disease and am on peritoneal dialysis.