I wouldn't worry about having a 3dB target noise margin, if the DLM isn't happy with line errors it will raise it to 6dB again within a day or two. The 21CN DLM tends to manage the line more aggressively than the 20CN one did. I think others have found leaving the router on 24x7 and letting the DLM get on with it is the best policy.
The absence of PPP session drops is encouraging - the improvement might simply be due to cleaning the contacts when the filters and cables were unplugged and plugged in again.
Upstream sync speeds slightly less than 448kbps are normal on ADSL2+ (even for LLU connections), presumably it's something to do with the technology.
Your upstream sync speed being (notionally) 448kbps whereas caulbox's is 1195kbps is not indicative of a fault. It depends how the service is configured, with capped 448kbps upstream as one option, uncapped the other. I understand fault rates on 21CN were significantly higher with uncapped upstream so bulk upgrades generally start with upstream capped. For stand-alone upgrades I think it is up to the agent, but capped is the default.
Plusnet will change ADSL2+ users to uncapped upstream if requested - this is free of charge. In your case I suggest requesting uncapping, but wait a few days to confirm disconnections really have gone away.
How long have you been on 21CN/ADSL2+?
David
