How long is it since you disconnected and reconnected? It's possible the high noise margin is just the result of conencting at a noisy time such as a thunderstorm. It might be worth a single re-conenction.
If the margin is around 12dB immediately after that, then you should get Plusnet to look into getting it reduced. It should do that automatically, by 3dB steps at intervals of 10-14 days if the line is stable and has low error rats, but that can fail and it sticks high.
You could get 832kbps now on ADSL if Plusnet support that. On ADSL2+ you will need to ask them to uncap it as they fix it at 448kbps by default, and unless they then cap it at 832kbps then you should connect at well over 1.2Mbps.
As has been said, Annex M will be of no real upload benefit and would almst certainly cost you over 2Mbps of download connection speed.
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