If the 3dB noise margin is from shortly after restarting the modem, and the connection speed is 4Meg, then suggestion is that you are now on 21CN WBC ADSL2+ and ISP/BT Wholesale has elected to give you a 3dB target noise margin to improve speeds.
In short its not ADSL2+ giving you more speed, but the way they manage the product.
62dB attenuation and 4Meg is pretty amazing, when it used to be said that the limit for even 2Meg was just 41dB.
The modem has been up to almost 8 hours and current connection speed is 3621kpbs. The 3500+ seems to normal for the last couple of weeks.
I didn't checked what the noise was last night when I got the 4 meg connection. Will remember to check when i get the 4 meg connection again.
I have to admit though netgear is a pretty solid router and I have rebooted it manually twice in the last year...
Also it must be a v3 router as I have V4.01.06 firmware which looks as though it was a v3 only firmware, infact there is a new version showing, might upgrade it later.
Thanks for your responses.