I got bogged-down making sure I had all the right codes in the right places so set up a few functions to return the service and characteristics, like this :
Then to read battery level on HRM and heart rate the calls look like this now:
Hope this is somewhat useful as it's from a huge project and just trying to pull out the useful bits.
I wished when I was working on this that I could use an XML GATT specification to automatically map on to the data but it was very manual to parse this stuff.
This was the hardest bit - trying to extract speed, cadence, stride length from my power meter!
I had a working invterval timer for running workouts but once I got my Polar Vantage M I didn't need my app anymore 
