I don't know why but I've now got internet access on my PC. I plugged all the ethernet cables (PC, home plug, PS3 and PS4) into the router and the network icon lost the yellow exclamation icon. I still didn't have internet access but, after rearranging the cables, I got access
I think that I might have been using the wrong cable before i.e. the PS4 ethernet cable, not the home plug ethernet
Of course, that doesn't explain why it didn't work when I connected the routers together.
Anyway, thank you to everyone who posted and especially you, Errkal. I hugely appreciate you taking the time and effort to help me troubleshoot this.
One thing still troubles me. In the network and sharing area of the control panel, my ethernet network is set up as a public network. How do I change this?