Have now received and fitted an Intel PCIe NIC, fully "plug & play" configuratioin, no issues.
The MoBo, on board Realtek LAN has been fine for over a week then yesterday for no apparent reason started to randomly not connect at very frequent intervals, had to be un-installed / re-installed to reinstate, un-installing always locked up the shutdown sequence and had to be down powered by force, driving me mad.
As per the earlier suggestions would have prefered to un-install the Realtek LAN in BIOS, however, there does not appear to be any way to do so on my Gigabyte MoBo, Legacy BIOS.
I can un-install via Win Device Manager but can't find any way of stopping automatic reinstallation at next re-boot.
Currently have to settle for disabling the Realtrek LAN which at least survives a re-boot and does not appear to be causing any network issues in that state, time will tell !
Edit, typo correction.