I've contact online tech support around 31 July re the above. Download speeds dropped from around 20mbps in Feb 2020, to around 11mbps in June, getting as low as 8mbps. In 2018 it got as high as 34mbps!
Upload speeds are now averaging around 0.3mbps compared to 7mbps in Feb 2020. Some noise on phone line which is intermittent. I can't test phone with a wired connection, I don't even know anyone who has such a thing anymore.
I've now left landline unplugged for past week or so and upload speeds are still poor and download ranges from 13-15mbps.
Disconnections have increased over the period and this morning from 8-9am I had no broadband at all.
It's now 19 August and I don't feel I am getting anywhere or any usable answers from Plusnet. It seems the emphasis is on 'it might be your fault' rather than be proactive. There have been regular issues before with exterior BT equipment (water leaking into the box & wires getting disconnected and just hanging). I've had the connection box inside replaced, and the line from BT pole to house replaced within past 2 years or so due to issues.
