So aside from a reduction in SNR which usually sits around 38-39 dB as memory serves, there's been a drop in power levels across the middle downstream channels. These would normally sit around -1 to -2 dBm since I am very close to the cabinet and on a 10 dB attenuator. There definitely seems to be a considerable drop in power from around the 187MHz - 363MHz band, though normal on the lower and upper downstream channels.
Modulation is clearly fine since upstream is still at 64QAM, and the 3.1 downstream is still at 4096QAM.
Cabling is finger tight and in excellent condition. I would say this is almost certainly a network fault.
I don't have another attenuator just the 10dB one, I'm pretty sure I heard some Virgin Media guys working outside a few hours ago (joys of shift work), and my BQM has been clear for a few hours, so suspect they've fixed some fault.
Power levels are still low, but currently getting the full 1Gbps and the SNR itself is high enough.
As long as the network is clear, I don't really care. But not everyone in my street is a network engineer constantly monitoring their own connection.