Ultimately I removed sources of interference from the vicinity of the ethernet cables and the hifi system. I mean the wifi router, a couple of switch-mode power supplies and an ethernet switch. What prompted me to do that was that I had my mobile phone near one of the ethernet cables one day (well away from the hifi though) and when it pinged the mobile network the RF was picked up by the hifi, amplified and sent out through the speakers. Moving the phone away from the cable made that stop. Once all this was done, the sound really improved when streaming over ethernet cables.
To be certain, I put those electically noisy items back where they were and the sound was degraded once more.
So, perhaps it's worth surveying the vicinity of the cable runs and the hifi for anything that could be injecting noise?
Robert