I got TSDuck compiled (took a while on the pi!) and with tsscan got below output for the range of frequencies
strength consistently in -53 to -46 dB range
SNR in 34 - 37,5 dB range
The BER is shown as 1%, but tbh I don't really believe that as when using tvheadend I never see any BER above zero in that UI
This is my reference point.
All I can really do is move the pi + card to other locations, connect to cable signal and re-run, and see if results are massively different.
Brad's Ubuntu drivers would probably need me to flip from my existing Debian installation, which I'd rather not do as I have some other stuff already running on the Pi. (Yes, I know Ubuntu is derived from Debian, but I don't want to make a mess mixing the 2 systems. But I do have a spare Pi somewhere, so I can also in parallel try to get that installed with Ubuntu and try).
Thanks for the help.
# tsscan --nit-scan --delivery-system DVB-C/A --frequency 330000000 -g
* Frequency: 330,000,000 Hz, locked: no, strength: -47 dB, SNR: 36.75 dB, BER: 1%
Transport stream id: 1051, 0x041B
* Frequency: 338,000,000 Hz, locked: no, strength: -52 dB, SNR: 35.5 dB, BER: 1%
Transport stream id: 1101, 0x044D
* Frequency: 346,000,000 Hz, locked: no, strength: -48 dB, SNR: 36.75 dB, BER: 1%
Transport stream id: 10019, 0x2723
* Frequency: 354,000,000 Hz, locked: no, strength: -48 dB
Transport stream id: 10016, 0x2720
* Frequency: 362,000,000 Hz, locked: no, strength: -48 dB, SNR: 36.75 dB, BER: 1%
Transport stream id: 3, 0x0003
* Frequency: 370,000,000 Hz, locked: no, strength: -51 dB, SNR: 35.25 dB, BER: 1%
Transport stream id: 10002, 0x2712
* Frequency: 378,000,000 Hz, locked: no, strength: -46 dB, SNR: 37.5 dB, BER: 1%
Transport stream id: 4, 0x0004
* Frequency: 386,000,000 Hz, locked: no, strength: -47 dB, SNR: 37.25 dB, BER: 1%
Transport stream id: 1, 0x0001
* Frequency: 394,000,000 Hz, locked: no, strength: -49 dB, SNR: 37 dB, BER: 1%
Transport stream id: 10018, 0x2722
* Frequency: 402,000,000 Hz, locked: no, strength: -50 dB, SNR: 34.5 dB, BER: 1%
Transport stream id: 10023, 0x2727
* Frequency: 410,000,000 Hz, locked: no, strength: -46 dB, SNR: 37 dB, BER: 1%
Transport stream id: 10020, 0x2724
* Frequency: 418,000,000 Hz, locked: no, strength: -50 dB, SNR: 36.25 dB, BER: 1%
Transport stream id: 10014, 0x271E
* Frequency: 426,000,000 Hz, locked: no, strength: -51 dB
Transport stream id: 10013, 0x271D
* Frequency: 434,000,000 Hz, locked: no, strength: -54 dB, SNR: 35.25 dB, BER: 1%
Transport stream id: 10012, 0x271C
* Frequency: 442,000,000 Hz, locked: no, strength: -54 dB, SNR: 35 dB, BER: 1%
Transport stream id: 10008, 0x2718
* Frequency: 450,000,000 Hz, locked: no, strength: -51 dB, SNR: 35.25 dB, BER: 1%
Transport stream id: 1079, 0x0437
* Frequency: 458,000,000 Hz, locked: no, strength: -47 dB, SNR: 36.75 dB, BER: 1%
Transport stream id: 1073, 0x0431
* Frequency: 466,000,000 Hz, locked: no, strength: -52 dB, SNR: 36.25 dB, BER: 1%
Transport stream id: 10000, 0x2710
* Frequency: 474,000,000 Hz, locked: no, strength: -51 dB, SNR: 35.75 dB, BER: 1%
Transport stream id: 10009, 0x2719
* Frequency: 482,000,000 Hz, locked: no, strength: -52 dB, SNR: 34.75 dB, BER: 1%
Transport stream id: 10010, 0x271A
* Frequency: 490,000,000 Hz, locked: no, strength: -52 dB, SNR: 35 dB, BER: 1%
Transport stream id: 10011, 0x271B
* Frequency: 498,000,000 Hz, locked: no, strength: -50 dB, SNR: 36.5 dB, BER: 1%
Transport stream id: 10006, 0x2716
* Frequency: 522,000,000 Hz, locked: no, strength: -48 dB, SNR: 37 dB, BER: 1%
Transport stream id: 6, 0x0006
* Frequency: 530,000,000 Hz, locked: no, strength: -50 dB
Transport stream id: 13, 0x000D
* Frequency: 538,000,000 Hz, locked: no, strength: -47 dB, SNR: 36.75 dB, BER: 1%
Transport stream id: 10003, 0x2713
* Frequency: 546,000,000 Hz, locked: no, strength: -50 dB, SNR: 36.5 dB, BER: 1%
Transport stream id: 10005, 0x2715
* Frequency: 554,000,000 Hz, locked: no, strength: -53 dB, SNR: 34 dB, BER: 1%
Transport stream id: 12, 0x000C
* Frequency: 562,000,000 Hz, locked: no, strength: -45 dB, SNR: 37.5 dB, BER: 1%
Transport stream id: 10017, 0x2721
* Frequency: 610,000,000 Hz, locked: no, strength: -47 dB, SNR: 37 dB, BER: 1%
Transport stream id: 10021, 0x2725