Ethernet switches, routers and cables are a known potential source of interference to reception. The effect was visible using the KiwiSDR on the 20 metre band with spikes every 60 kHz.
To mitigate this interference source, the switch was changed from a TP-Link unit to a Netgear GS305. It was hoped that having a switch in a metal case would be better than one in plastic. In addition, all LAN cables were changed to shielded CAT6 and the switch placed 20 metres away from the receivers.
A subsequent check displayed significantly reduced interference.