Unless PlusNet are somehow thottling the throughput due to putting them on the wrong package. Good chance this is in PlusNet land rather than the connection.

My 5p is on this being in the plusnet area of problems.

The turning off and leaving off for 30 minutes might trigger a relearning of the settings on the Plusnet side.

Possibly, turning off an leaving nothing connected for a reasonable time should cause any sessions to be terminated and then when reconnected it may bring a new set of correct parameters into play. If doing this does not resolve it, then the ISP needs a kick.

Followed MHC's procedure to the letter with 80-minute switchoff and wifi off, but it makes little difference:
TBB speed test

The new router stats are:
DSL uptime: 0 Days, 0 Hours 2 Minutes 31 Seconds
Data rate: 10.00 Mbps / 40.00 Mbps
Maximum data rate: 34871 / 88255
Noise margin: 22.7 dB / 19.6 dB
Line attenuation: 17 dB
Signal attenuation: 12.1 dB / 17 dB

Many thanks again for your advice, as others suggest I'll try posting on the Plusnet forum which seems quite active these days.

I suspect that your on the 40/2 product then, might need to ring them and ask for a regrade

Should point out, that the Openreach kit has the user on the correct 40/10 product, so any changes will likely just be at the Plusnet end, i.e. no need to issue a regrade order with their suppliers

To me that proves it to be in the Plusnet arena. Sync speeds are fine for the line, Max rate, much the same. Speedtest suggests a cap somewhere and time to nudge Plusnet.

TBB experts come up with the answers yet again! Many thanks everyone, I shall contact PN and let you know how I get on.
Best wishes, stay safe! Mike

This is an odd one, you are obviously on a 40/10 product but your upload speed is very low. As far as I am aware, and I have been with PlusNet on and off for many years I have not heard of a PlusNet upstream line profile, they do have a downstream one, called current line speed which sometimes gets stuck and causes problems. Never known of a similar problem with upstream. You may have trouble getting a sensible response from them as their customer support is struggling in the present situation and they are prioritising customers with total loss of broadband or phone and saying speed problem investigations may be delayed. Hope you can get it sorted.

They did initially sell the 40/2 product.

It could be that although they have been correctly provisioned on 40/10 that some other Plusnet setting that we don't see has been made at 40/2.

The provisioning and the "account control" (not "Accounts") department are separate.