Beats Fit Pro and Apple Watch connection cuts out after a few seconds

I have seen various posts on this topic but can't find a decent solution. I have brand new Beats Fit Pro and a Series 7 Apple Watch. I have lots of music downloaded on my music that I listen to when I go for a run. ie without my iPhone. However when connecting just to the Apple Watch (with bluetooth on my phone turned off) a track will only play for a few seconds at 3/4 speed (it is noticeably slower) and then the sound completely stops although the track is still playing on my watch.

It is totally infuriating as I purposely bought these headphones for running in and they do sound great, but it seems pointless if I have to take my phone with me at all times to work.