My parents used to be on 3dB for a few months then it changed to 4dB+ for a long time (2 months) then it get worse 5dB then back to 6dB and stay there for more than 3 months and their 80/20 has dropped to low 47/19 and they downgraded to 40/10. They don't need 80/20 anymore.

You might end up the same as my parents.

I'm hoping that mine will recover, seeing as it was due to repeated power cuts, rather than inherent line conditions.

It'll be interesting to see what the periodic BT hub reset brings... every 14 days, isn't it? It's been connected for 12 days now since the last power cut, things seem to be okay again.

14 day reboot of the hub last night and back to 3dB.

Result!