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3x TP-Link switches, 1 TP-Link Range Extender (access point mode), plus the usual phones, tablets, computers, TVs, gaming consoles, NAS storage etc.
Thanks Majanzo.
Your message has reminded me we also have Arlo security cameras on the network. The base station gets an IP from our Archer router, but it has it's own wifi (and DHCP, I presume) which the cameras connect to. It is effectively a black box that I can't configure. That is the only device I can think of that might be assigning IP address which conflict.
So I have tried changing the IP address range for my Archer router to see if that removes the conflicting addresses and resulting issue. I'll see how it goes over the coming 6 weeks to see if this resolves the issue.
Thanks again