I have been using a very stable combination of a flashed BT Infinity modem and a Billion 7800N for a long time, the modem stays up for months ( as does the Billion) without an issue with the added advantage of the stats being available. Our place has internal walls of solid concrete blocks ( not those light fluffy things), ceilings are of reinforced concrete yet the Billion's wifi reaches everywhere, even outside. The equipment is in a cupboard with a hefty wooden door also, a testament to the wifi capability I think.

As you have sensibly used ethernet everywhere, why not install a wifi repeater/s somewhere that can extend the reach into the dead areas?

I'm not recommending the Billion 7800N to you as it's wifi is now a bit dated, particularly if you want to look toward streaming 4K, but I heartily recommend the brand in combination with a flashed modem.

In reply to a post by gt94sss2:

BT are expected to release their new Smart Hub as a successor to the HH5 - and most assume it will feature upgraded WiFi - so you may want to hold off until that is released before you make a decision (as they tend to be much cheaper than 3rd party models)

uSwitch are saying BT will announce the HH6 next week - http://www.uswitch.com/broadband/news/2016/06/bt_to_...

Hi - will i need to flash/unlock the modem? Or is that just a nice to have in order to get access to line stats via telnet?

In reply to a post by Wonky:

Hi - will i need to flash/unlock the modem? Or is that just a nice to have in order to get access to line stats via telnet?

Flashing the HG612 with unlocked firmware will allow you to see stats, however the modem will work fine without flashing. I've flashed mine with the 'webgui' version which allows me to see basic stats via web browser and more detailed stats via telnet
http://www.kitz.co.uk/routers/hg612unlock.htm