Apple Watch series 6 data no longer showing on iPhone 6s Fitness or Health apps

Got Apple Watch series 6 10 days ago, sync'ed to iPhone 6s and it has worked well until yesterday. Yesterday the data on the Fitness app did not reflect the watch - at least an hour out. I tried switching both off and on again, but no change. Later in the day, still no change and I noticed an update for both was available which I applied. This made no difference. This morning when I switched the phone on data appeared in Fitness, but not only for a short while , then it disappeared. I don't have cellular, and both wifi and bluetooth are working on both devices. I have been changing some of the settings to improve battery on watch which last week was not lasting 24 hours, but not aware of any I could have changed that would have stopped syncing. Any advise on settings to change on either that will get the syncing working. I have a lot of data now on my watch that is not on my phone and can't be backed up. Looked at unpairing and re-pairing but it looks like you lose all the watch data doing that?