Well this was unexpected. The banding on my Plusnet line got removed which is allowing the HH5a with the tweaked SNRm to sync as follows:
Chipset: Lantiq™ XWAY™ VRX268
Firmware Version: 5.7.9.9.0.6
API Version: 4.17.18.6
MEI Version: 1.5.17.6
Power Management Mode: L0 - Synchronized
Line State: UP [0x801: showtime_tc_sync]
Line Uptime: 7d 19h 18m 3s
Resyncs: 3
DSLAM/MSAN VID:
XTSE Capabilities: 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x2
Annex: B
Line Mode: G.993.2 (VDSL2)
Profile: 17a
Trellis: D: ON / U: ON
Bitswap: D: ON / U: ON
G.INP: D: Enabled / U: Not Enabled
Virtual Noise Support: D: Not Supported / U: Not Supported
Attain Data Rate: 77.079 Mb/s / 18.557 Mb/s
Actual Data Rate: 73.909 Mb/s / 20.000 Mb/s
Impulse Noise Prot: 43.6 sym / 0.0 sym
Interleave Delay: 0.17 ms / 0.0 ms
NFEC: 32 / 255
RFEC: 16 / 16
LSYMB: 16 / 5410
Interleave Depth: 1 / 1
Interleave Block: 32 / 255
LPATH: 0 / 0
Line Attenuation: 11.5dB / 12.2dB
Signal Attenuation: 11.5dB / 12.0dB
Noise Margin: 3.1dB / 4.4dB
Transmit power: 14.9dBm / 1.9dBm
FECS: 0 / 53330
ES: 229 / 4007
SES: 7 / 51
LOSS: 2 / 0
UAS: 212 / 212
HEC: 0 / 0
CRC_P: 0 / 0
CRCP_P: 0 / 0
15m Code Violations: 0 / 0
15m FEC Errors: 0 / 4
1d Code Violations: 396 / 142
1d FEC Errors: 0 / 801
It took a few days, but the Plusnet line profile updated too so my Plusnet line amusingly is now running faster than my Zen, even though my Zen line has better stats (but is still banded).