Chromecast and Technicolor router

[#1]

I'm having some fun (!) with my Chromecast which arrived courtesy of Santa this year.
My setup is a Plusnet router, configured to use the OpenDNS servers rather than PlusNet default, combined with a separate router (custom firmware etc, call this Router B) with same SSID acting primarily as a hard switch (DHCP switched off, static IP assigned to the router) connected via TP-Link Powerline adaptors, supplemented by a TP-Link wifi powerline extender in another room.  All necessary (ish) to ensure good wifi coverage around the house.
My normal setup would have Router B with no wifi, as it serves mainly to distribute several ethernet connections to devices such as my digibox, Xbox and TV.  But it is co-located with the TV.
When I initially tried to setup Chromecast, it wouldn't connect, so I faffed around a bit and found that enabling the Wifi on Router B gave the Chromecast something to connect to.  I assumed that wifi power around the TV was interferred with, preventing either the normal PlusNet router or the Wifi extender from serving wifi to the Chromecast.  However, I then remembered why I had disabled Wifi on Router B, and it was because (for reasons unknown, and irrelevant to this discussion) it caused problems around the house for some devices.  So in an ideal world, I'd like to switch off the wifi from Router B and only have Wifi served from the plusnet router and from the wifi extender.
So I played around a little.  I disabled wifi on Router B and found that the Chromecast still was showing a connection (I could tell this was the case as it was successfully streaming my photos in "background" mode etc).  I then disabled the wifi extender, and Chromecast was still working fine in terms of being connected.  But when I did this, I could no longer connect to Chromecast from any phone / tablet / PC etc.  So, despite both the Chromecast and the phone etc both being connected to the PlusNet router, being on the same SSID, and the router successfully showing both devices connected, I couldn't seem to connect from the phone etc to the Chromecast.
So it would appear that there is something in the combination of the Chromecast and the Plusnet router which aren't playing nice together.
I don't believe there is a visible option to turn on or off AP Isolation / Guest Mode, but I have noticed a couple of times that when my phone has connected to the PlusNet router it has displayed the same sort of message that I'd get if I went into McDonalds etc - "Sign into Wifi Network".  Only sometimes, however, so this could be a red herring.  Is it possible that AP Isolation is toggling on / off, or is incorrectly configured somehow? 
I'm comfortable with telnetting into the router to change / view settings, but as others seem to successfully use Chromecast with the Technicolor router, I was wondering if there might be any other avenues to explore?  Perhaps my manual override of the DNS servers is something of a cause?
Any help gratefully received!!  Sorry if this sounds very convoluted!! 

[#2]

Why have duplicate SSid's?
Is more than one router providing DHCP services?

[#3]

It's so the whole house is on one SSID and devices (particularly phones / tablets etc that move around) don't end up dropping off the network when they are too far away from the router that they originally made contact with.
I was under the impression that maintaining a single SSID for a whole building was the preferred way of doing things (certainly that is the default mode for the TP-Link wifi extender).
The only device which has DHCP enabled is the plusnet router, both of the other devices have DHCP switched off. 
My preferred mode is to only have two devices broadcasting the SSID - the plusnet router and the TP-Link wifi extender.
[Moderator's note by Adie (dvorak):  Full quote of preceding post removed, as per Forum Rule] <<Thanks Mod!  Daft on my part!

[#4]

I've just never needed/wanted to be that mobile!

[#5]

It's partly a function of the house layout, not really designed for the modern world!  Router is in front left of the house, with the BT socket, several devices need / want to be in the back left.  And then we've got an attic room etc etc.  Three access points is overkill, and not all devices roam across wifi access points as nicely (e.g. my phone disconnects / reconnects in about 0.5s, whereas my Hudl 1.0 takes about 10s to switch from a less powerful to a more powerful access point with the same SSID).
I'm sure someone handier than I am with tools could work out a way to get our plusnet router better located for whole-house wifi propagation, but for now I'll have to rely on my IT skills to keep us all connected instead!