Soooo after a fair few attempts I noticed that the second h9 (it mattered not whether it was my original h9 or my new h9) would hold connection for 5 seconds, then a bit longer, and so on.
Eventually it appeared to stamilise and stay connected. Thereafter I could open h9 control and switch between h9’s as it’s meant to.
I’ll test tomorrow but I’ve noticed the initial Bluetooth connection has stabilised then it’s usually fairly robust. If any issues I’ll add to this thread tomorrow in case it helps others with similar issues.