Appoligies for the radio silence - but it looks like I solved it.
So I have a win 10 PC which acts as a server on my home (ethernet) network, connected via a TP-Link TL-SG1016D 16-Port Gigabit Ethernet Switch. Actual Data is on a Zyzel 542 NAS and my router/modem is a BT Superhub 2 (latest BT).
I moved to a house that is fully cabled up with cat 5, but it didn't work, so I was using powerline until it was fixed. Finally moved to the fixed ethernet - fantastic speeds etc. Then gradually i started having connection problems - the biggest one being Emby.
Long story short, I wiped the server and reinstalled Win 10 and Emby and voila! all back to normal. Only one issue - not an Emby one, but you may come across this from 'a little knowledge is a dangerous thing' types like me (and someone here might be able to help).
My server (Win 10) says it is connected to the network but I cant connect to anything unless I fire up my VPN and then ... everything appears and connects.
I'm guessing I have some sort of IP address issue on Windows or on the BT router, which is overridden by the VPN (VPNord) handing out its own IP and this was the same issue originally.