So the engineers turned up today and did some more investigation, they claimed our line build (whatever that is) was set to 40/10 and that they will have a new build that fixes it at 80/20 before the line resets to 40/10 again.
I've not heard of line builds before, but what I can see is that the HG612 is now showing the following stats:
xdslcmd info --stats
xdslcmd: ADSL driver and PHY status
Retrain Reason: 1
Last initialization procedure status: 0
Max: Upstream rate = 21421 Kbps, Downstream rate = 73608 Kbps
Bearer: 0, Upstream rate = 19999 Kbps, Downstream rate = 73839 Kbps
Link Power State: L0
Mode: VDSL2 Annex B
VDSL2 Profile: Profile 17a
TPS-TC: PTM Mode(0x0)
Trellis: U:ON /D:ON
Line Status: No Defect
Training Status: Showtime
SNR (dB): 6.1 7.4
Attn(dB): 16.4 0.0
Pwr(dBm): 13.6 7.0
Strangely our sync rate is actually marginally higher than the attainable which is odd, but G.INP isn't currently active so I suspect that will come into play soon.
I wasn't around but my brother did mention the noise issue to them, they tested for radio noise and apparently the Sonos bridge was causing tons of noise - so looks like I'll be moving that away from the modem. That may have been the cause of the QLN bump in D3 but we'll see.