Bit the bullet and did a clean install of 12; everything definitely working as expected now - haven't gone backwards; it actually fixed and error notification I used to get after startup whereby the PVR-HTS addon would report an invalid response from the TVHeadend server hosted on this same device - this now connects straight away without error
The behaviour of having to reboot if you restart your router or take the Ethernet cable out is still there, you get the Input/output error I reported earlier if you try to refresh or connect - this was there in 10 and 11 so not new