Noise margin doesn't really change. It's using the brightbox router. I had this replaced about a month ago and still the same problem. Do you think it's to do with that manufacturers router?

Most people do fine with the brightbox, so it may just be that your line is not handling a 3dB target margin very well.

Orange for a long time had a habit of slower syncs than other providers, and recently a few lines showing up that may be pushing speeds too far.

A revert to a standard 6dB target margin, might reduce errors and drop a bit of sync speed but give a much better overall experience. Of course you can try experimenting with wiring, different routers etc to try and reduce the errors

I doubt it is the Bright Box router at fault. Over the past two weeks I've had problems with my line, suddenly my lovely upstream speed 1249 Kbps did a dive down to 440 Kbps and at times BBC iPlayer would not run.
I have been monitoring the connection and to my delight the upstream speed is increasing, it�s now this morning at 888 Kbps that�s some relief. I was thinking an engineer had done a (lift and shift) and used my connection.

ADSL Status
This page shows information about your ADSL connection if applicable.
Status
Configured Current
Line Status --- SHOWTIME
Link Type --- Interleaved Path
Operation Mode Automatic G992.5(ADSL2+)

Data Rate Information
Stream Type Actual Data Rate
Upstream 1249 (Kbps.)
Downstream 13365 (Kbps.)

Defect/Failure Indication
Operation Data Upstream Downstream
Noise Margin .... 6.1 dB 5.9 dB
Line Attenuation 15.4 dB 29.0 dB

Indicator Name Near End Indicator Far End Indicator
Output Power 12.4 dBm 0.0 dBm
Fast Path FEC Correction NA NA
Interleaved Path FEC Correction 40286 0
Fast Path CRC Error NA NA
Interleaved Path CRC Error 273 0
Loss of Signal Defect 0 0
Fast Path HEC Error STR NA NA
Interleaved Path HEC Error 7033 0
Error Seconds 9 0
Statistics
Received Cells 118598
Transmitted Cells 20153
-----------------------

ADSL Status
This page shows information about your ADSL connection if applicable.
Status
Configured Current
Line Status --- SHOWTIME
Link Type --- Interleaved Path
Operation Mode Automatic G992.5(ADSL2+)

Data Rate Information
Stream Type Actual Data Rate
Upstream 440 (Kbps.)
Downstream 13298 (Kbps.)

Defect/Failure Indication
Operation Data Upstream Downstream
Noise Margin .... 25.6 dB 1.3 dB
Line Attenuation 15.5 dB 29.5 dB

Indicator Name Near End Indicator Far End Indicator
Output Power 12.3 dBm 0.0 dBm
Fast Path FEC Correction NA NA
Interleaved Path FEC Correction 47429 188
Fast Path CRC Error NA NA
Interleaved Path CRC Error 518 2
Loss of Signal Defect 1 0
Fast Path HEC Error STR NA NA
Interleaved Path HEC Error 14471 4
Error Seconds 407 75
Statistics
Received Cells 497450
Transmitted Cells 33649
-----------------------

ADSL Status
This page shows information about your ADSL connection if applicable.
Status
Configured Current
Line Status --- SHOWTIME
Link Type --- Interleaved Path
Operation Mode Automatic G992.5(ADSL2+)

Data Rate Information
Stream Type Actual Data Rate
Upstream 888 (Kbps.)
Downstream 13264 (Kbps.)

Defect/Failure Indication
Operation Data Upstream Downstream
Noise Margin .... 16.2 dB 6.4 dB
Line Attenuation 15.5 dB 29.0 dB

Indicator Name Near End Indicator Far End Indicator
Output Power 12.3 dBm 0.0 dBm
Fast Path FEC Correction NA NA
Interleaved Path FEC Correction 5244 0
Fast Path CRC Error NA NA
Interleaved Path CRC Error 53 0
Loss of Signal Defect 0 0
Fast Path HEC Error STR NA NA
Interleaved Path HEC Error 1616 0
Error Seconds 31 0
Statistics
Received Cells 585769
Transmitted Cells 95872

Do you think the upstream SNRM will continue to reduce over time since previously with interleaving you got a 1249K sync?

Also isn't a 13264K downstream sync still a little slow on a ~6dB SNRM and 29dB attenuation - perhaps one would expect ~16000K ?

It is usual to get 888K Up on Interleaved. Indeed it was surprising that you ever got higher.

AS I've pointed out to you previously you should be getting about 16 Meg Down.

The 2nd set of stats has a v. low Down NM.