Ok, well I am back to provide an update. After posting here, referring to the mods and eventually today having an Openreach engineer out to look at the problem, all my woes have gone.
I'd like to offer a huge thanks to everyone who helped, and although he won't read this, you Mr Openreach Engineer are a genius!
So, it turns out the line was far too noisy. It wasn't down to internal cabling at all but the copper pair I was on between the cabinet and the exchange.
In my house, the engineer moved the NTE5 master socket to the room where the router lives, and he also installed an Openreach filtered faceplate. I'd just ordered a whole load of new ADSLNation microfilters too so they're now redundant! Anyway, that change alone got me onto ADSL2+ rather than ADSL2, and the S/N margin was much lower.
He then went out to check the pairs available between the cabinet and the exchange and has moved us onto a different pair. Check out my router stats:
ADSL Line Status
|Connection time:||0 day, 00:03:48|
|Modulation:||G.992.5 Annex A|
|Noise margin (Down/Up):||5.9 dB / 5.8 dB|
|Line attenuation (Down/Up):||35.5 dB / 19.0 dB|
|Output power (Down/Up):||0.0 dBm / 12.9 dBm|
|FEC Events (Down/Up):||0 / 0|
|CRC Events (Down/Up):||41 / 0|
|Loss of Framing (Local/Remote):||0 / 0|
|Loss of Signal (Local/Remote):||0 / 0|
|Loss of Power (Local/Remote):||0 / 0|
|Loss of Link (Remote):||0|
|HEC Errors (Down/Up):||529 / 0|
|Error Seconds (Local/Remote):||0 / 0|