Quick update!
I tried 2471201 as Voodle suggested - it seems to be the one to help with longer lines that I was looking for, Thanks!

SNR Margin is now 11dB, and Loop Attenuation is now 7dB.

The router seems to be trying its best, but after the update the DSLAM has seen a disconnect and is punishing me further by ramping down the sync speed.
I'm going to leave it to stabilise for a week or two, and if it doesn't slowly increment up to a better speed, I'll call my ISP and they can take a look at the profile and caps (if it seems to be artificially low).

Thanks for the help, and anyone with a long line may want to give 2471201 a try - 7dB Loop still makes me lol a bit - the DSLAM must think I'm in the next rack...