Fair enough!
Fwiw it turned out to be a negotiation issue on my switch rather than the NIC. Fixed it to 2.5G and have a solid link on the latest Unraid release.
Hey folks - @ThatDude especially,
I've blacklisted the r8169 driver as above, and looking at the logs appears to show the correct driver being loaded:
r8125 2.5Gigabit Ethernet driver 9.005.01-NAPI loaded
However, I'm only seeing a 1G link. Were there any other tweaks that were required?