Just an update in case anyone is interested, I seem to have fixed the problem with a setting change.
After settling on the solution to get a powerline adapter with wifi built in I ran into some money issues and put the whole thing out of my mind.
However the lockups were becoming regular, pretty much once a day after work, so instead of just rebooting the router last time I started delving around the settings in the TP-Link router.
Feeling a little foolish now, especially if this stays stable but it's been over a week so far and all good.
What I found was DHCP relay was on, so I turned it off, and the network immediately sprang into life, no reset needed.
I'm not sure why it should cause issues, seems the purpose of it is to pass on DHCP requests etc between different subnets. It shouldn't mess up the network, but it does?
Anyway I only have the one subnet so no need for a relay. I don't know how it came to be on but it was.
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