After setting up the scale ok I stepped on for initial weigh in, no body composition measurements, only weight and a big 'X'. This turned out to be a firmware issue and it fixed itself after about measurements and 3 resets.
Then.. The body composition measurements were clearly wrong. The garmin had me at 6% body fat which is on Usain Bolt level which I'm clearly not. I compared these to my other tanita scale which gave 14% which sounds much better. I followed the garmin instructions for when to measure (ie not first thing in the morning, after sauna, etc) and i tried over a number of days at different times.
Contacted garmin support with all the information they needed and all they could offer me was to point me to their measuring instructions which I'd already followed. They said they weren't seeing any similar problems but I did find similar stories online.
This is garmins first scale and looks like they need more time to refine their product.