I was discussing this with a friend today. Retired now but from the industry, he reflected that call centers have no technical knowledge and are very rarely confronted by such a regime of constructively linked questions! So they won't bother replying, or if they do the response content would be worthless? However he suggested moving the present router to the loft and adopting an ethernet connected extender or two to boost the signal to the properties extremes, as you had suggested. However to do that I'd have to alter the present incoming land line set up.
I suppose I could always drill through the exterior wall and run the phone out from its existing socket up the wall and directly into the loft under the eves - total distance 10 Ft maybe? He also suggested investing in a router replacement at this stage, as my 3 year old 'talktalk' supplied hardware would be easily bettered today?
I'd then retain the existing landline for a nett cost of £6.00 a month and use the mobile if ever I have to make a call? He thought it unlikely TalkTalk would want to try and up my charges just now anyway?
He did also mention that extending a 4G signal, even with an ethernet booster might only marginally improve an any way inferior wireless originated signal, comparison to the existing 35mbps landline set up? Adding also that I shouldn't conflate dongle 3G signal reception with that from a router, which he considered would be inferior. 'You only discover performance post committal' he warned!
The physical work these days however, is beyond me and would mean getting a contractor. I used to know a BT fitter who would have facilitated this for a reasonable price and quickly, but that was in London. I suppose its worth seeing if there's anyone locally, or even requesting BT for a price to provide a second extension point to the loft, for a pro it can't be that difficult. The rest I could complete with the aid of a neighbour?
Thanks for reading this, I hope our exchanges will help anyone else considering this change and for that purpose I'll publish further events.