check host connectivity from vCenter to Host and also telnet port 902
Traced the error. Finally. Lots of log reading.
On vCenter in the vpxd log I was getting:
2020-04-06T11:10:04.152+02:00 warning vpxd[7FE0B1932700] [Originator@6876 sub=InvtHostCnx opID=HeartbeatStartHandler-7eda5023] [VpxdInvtHost] IP address change detected from 188.8.131.52 to 184.108.40.206
After a LOT of digging, came across this article: https://kb.vmware.com/s/article/1035944
I got one of our network guys involved, turns out there was a NAT rule on the firewall had been left running on a vpn between the two sites. Disabled NAT - was supposed to be off anyway - and the host connected and stayed connected with no further issues.
Never come across this before - new one on me. Might help someone else.