I just thought I'd update my old thread, effectively closing it as "SOLVED".
Since around 20th December 2017 onwards, the speeds obtained from my load-balanced ADSL + 4G link have been very respectable - ranging from 10Mbps down & 1Mbps up, more recently to just over 50Mbps down & over 10Mbps up (at times)! Only on 1 day has the speed dropped below 10Mbps down (& then, only to 8Mbps down) - other than that, my 4G connection has fulfilled my ambition as a "DIY USO [Universal Service Obligation] connection", to provide me with a turbo-boost (and backup) to my slow ADSL Max link to boost it to at least 10/1.
I can only assume that, during last autumn, EE were doing work on & upgrading the cell masts in my local area (possibly even provisioning extra ones) - hence the erratic connection quality during that time. Of course customers seem to be kept "in the dark" about work on local cell towers, hence my confusion over the degraded performance. The EE network coverage map now shows "excellent indoor and outdoor" 4G coverage, and also "Double speed 4GEE", for my area, which is a very recent change (used to show as "weak outdoor & no indoor" coverage) - so I assume any work was related to this!
The "LTE Discovery" Android app seemed to indicate a possible new cell tower nearby - so I re-positioned my router in another upstairs window (where there was previously no 4G signal), and this yielded my current average downstream speeds of > 30Mbps (sometimes over 40Mbps or even 50Mbps+), and upstream speeds of consistently over 2Mbps, sometimes over 10Mbps. (e.g. SpeedTest.net result
). Much better! And that's still without using my external antenna - which will be installed & tested soon.
Now all I could do with is a large data allowance on SIM-only (EE do 200Gb on Mobile Home Broadband, but only if you use their own router).
Hopefully we'll soon get Fixed Wireless link too in our area (or even maybe FTTC/FTTP one day!). But until then, at least 4G seems a good stop-gap.
Anyway - "Case Closed"
Many thanks for all ideas & advice received in this thread.