Trouble setting up Asus RT-N66u router

[#1]

The Netgear settings seem fine to me for the double-NAT configuration you have chosen to use.

Odd that you cannot get the Netgear to work in bridge mode with the Asus.  PPPoE only works with LLC-based encapsulation on BT wholesale network.  It simply won't work with VC-mux. 
Fwiw, if a previous PPPoE session has not been cleanly closed down, I believe there is a Plusnet 'feature' which requires users to wait 2-3 minutes before a new PPPoE session can be established by the router. This doesn't affect PPPoA connections.

[#2]

I don't know if "chosen" is the word I'd use!  I'd prefer not to go the double NAT route but my equipment's not playing ball for some reason
What I find really odd is that the modem only configuration will work with the PC connected directly, but not if I insert the Asus between the two.  Any ideas why that might be the case?
I read on another thread that you should change the "ISP requires a Login" to "No" in the Netgear but it was a slightly different issue and I didn't want to mess about too much with things I didn't understand.  Would you recommend?
I know when I change / disconnect it takes a good few minutes before the connection is recognised, which I'm assuming is what you are referring to.  I've waited at least 5 to be safe before trying but didn't get any joy.
I may have misunderstood you but I couldn't get PPPoE to work at all.  My modem is connected under PPPoA?  With the Asus the only configuration that worked was using the Automatic IP setting, and that was when the Netgear was set as a modem/router.  When I had it as a modem only the icon in the bottom right corner of the PC (on both my wired PC, wireless laptop and my mobile phone) told me I had internet access but I couldn't actually connect to anything.
I appreciate the help on this

[#3]

When you had PC directly connected to the Netgear in bridge modem mode, can you confirm were you successfully using PPPoE on PC and LLC encapsulation on Netgear?
Btw, in bridge mode, you should not be using/storing any username or password in the modem setup.  The PC (or Asus) handles the login authentication.  ie.  "ISP requires a Login" to "No".  
In bridge mode, the connection type should NOT be set to PPPoA, or even PPPoE in my experience of using other routers in bridge mode, try Dynamic IP, DHCP, or whatever it may be called if offered in the Netgear firmware.

[#4]

Hmm, maybe the login was messing it up then.  That might explain why it worked when connected directly to the PC but not when connected to the router.  I will check that later, thank you.
I'm out so can't do the screen dump yet.  From memory it was working with PPPoE when connected directly to PC, but pretty sure it was still VC-based, not LLC.
Basically when I switched to modem only mode I didn't change any other settings, so all the login details remained.
Appreciate the help and will give it another try later

[#5]

Success!  It was the login information in the end.  I'd assumed that the modem handled all the inbound traffic and the router just distributed it.  Now have Netgear set up in modem-only mode, no login details and LLC-based.  Asus logged in over PPPoE.
Thank you so much for your help