Difference Between Upstream and Downstream SNR

[#1]

Current router ADSL2 stats are as shown below:
Upstream 1127
Downstream 4672
SNR Margin(Upstream) 6.4
SNR Margin(Downstream) 16.0
Line Attenuation(Upstream) 21.6
Line Attenuation(Downstream) 36.5
Can anyone suggest where to start investigating why the downstream SNR is so high and how to resolve the issue.  There have been several power cuts each week for the last month or so and I think this may have lowered my line profile, currently reporting as 4.12Mbps.  Prior to the issue with the power the downstream SNR was almost the same as the upstream value and the downstream speed was at least 8Mbps. 
Power issue have now been resolved  - does the IP profile just need resetting?

[#2]

The DLM (Dynamic Line Measurement) appliance in the exchange has probably registered the disconnections due to the power cuts as symptoms of a dropping connection and raised your downstream target SNR margin to 18dB (the maximum) to try to stabilise the line. This has resulted in the much reduced sync (line) speed.
If the connection is now stable with low error rates the DLM should, in time, reduce the target SNR margin again. Since the power issues have been resolved Plusnet may agree to reset the connection to speed this process up. Resetting will put the line into training mode again.

[#3]

The important thing at the moment is DO NOT REBOOT YOUR ROUTER. If Plusnet can see a long continuous connection they are more likely to agree to do the SNR reset when you ask.