I arranged a call through the help desk. I had already unpaired and re-paired. Turned my phone on and off. Had the latest software. The guy (Eddie) was really helpful. I had to turn off a new setting of the 5series (that I now can’t find in my app), something to do with always on, might have been wake screen always On?) within 12 hours my watch switched itself off again and I thought, here we go again! Glad to say it has been fine ever since (3 weeks ago).
Don’t charge your watch on non-iPhone chargers like generic night stand docs. Arrange a call with the helpdesk. I set it up the night wrote for 3 pm the next day and they called at 3.05 pm and it was all done within 10 mins. I thought it hadn’t worked when it turned off again the next day but it hasn’t happens since. Good luck