The Downstream Power level has a preferred range of -7 to +7 dBmV however...

3.0 Downstream channels 28 to 31 are well below this at -10.8 & -10.9 dBmV

3.1 Downstream is considerably lower at -21.9 dBmV

I suspect a local street cabinet issue is the problem, if not your are going to need an engineer to fix.

Check for known faults in your street. Try both methods:

Call the local fault line: 0800 561 0061 – this is an automated number.

https://www.virginmedia.com/help/check/status/identification/identify

The highest frequency channels are losing the most signal, which usually suggests a cable fault somewhere between the street cabinet and your Hub. The first thing to check is that the coax connections inside the house are tight and the cables are not damaged. If those are all secure then you will need to phone CS and report the fault. 

Alessandro, thank you for your advice.  My original Hub 3.0 required an attenuator to be fitted inline at the Hub as the SNR was too high all those years ago. I've left this in place - could this be the issue? 

That is of course the obvious thing to do.  Is there a page specifying the levels (or is it simply a range of -7 to +7 dBmV for power levels).

I removed the attenuator at the Hub 5,0 input.  I now get this which looks better but I'm still not sure it's right. All QAM64 and PostRS errors are not that close to zero in some cases. 

3.0 Downstream channels

Channel    Frequency (Hz)    Power (dBmV)    SNR (dB)    Modulation    Channel ID
1    194000000    7.9    39    QAM 256    8
2    138000000    10    39    QAM 256    1
3    146000000    9.5    38    QAM 256    2
4    154000000    9    38    QAM 256    3
5    162000000    8.7    38    QAM 256    4
6    170000000    8.4    39    QAM 256    5
7    178000000    8    39    QAM 256    6
8    186000000    7.9    39    QAM 256    7
9    202000000    7.4    40    QAM 256    9
10    210000000    7.2    40    QAM 256    10
11    218000000    7.2    39    QAM 256    11
12    226000000    6.7    39    QAM 256    12
13    234000000    5.9    40    QAM 256    13
14    314000000    3.5    40    QAM 256    23
15    322000000    3.4    39    QAM 256    24
16    330000000    3.2    39    QAM 256    25
17    338000000    2.7    39    QAM 256    26
18    346000000    2.9    39    QAM 256    27
19    354000000    2.8    39    QAM 256    28
20    362000000    2.2    38    QAM 256    29
21    370000000    1.8    38    QAM 256    30
22    378000000    1.6    38    QAM 256    31
23    386000000    1    37    QAM 256    32
24    394000000    0.5    39    QAM 256    33
25    402000000    0.4    39    QAM 256    34
26    410000000    0.2    39    QAM 256    35
27    418000000    0.4    39    QAM 256    36
28    522000000    -4.8    36    QAM 256    37
29    530000000    -4.7    36    QAM 256    38
30    538000000    -4.6    37    QAM 256    39
31    546000000    -4.7    37    QAM 256    40
3.0 Downstream channels

Channel    Locked Status    RxMER (dB)    Pre RS Errors    Post RS Errors
1    Locked    39    0    0
2    Locked    39    12    0
3    Locked    38    15    0
4    Locked    38    17    1
5    Locked    38    3    0
6    Locked    39    0    0
7    Locked    39    0    0
8    Locked    39    0    0
9    Locked    40    0    0
10    Locked    40    0    0
11    Locked    39    2    0
12    Locked    39    0    0
13    Locked    40    5    0
14    Locked    40    1    0
15    Locked    39    2    0
16    Locked    39    2    0
17    Locked    39    1    0
18    Locked    39    28    0
19    Locked    39    3    0
20    Locked    38    5    0
21    Locked    38    7    0
22    Locked    38    7    0
23    Locked    37    6    0
24    Locked    39    15    0
25    Locked    39    12    0
26    Locked    39    17    0
27    Locked    39    16    0
28    Locked    36    176    0
29    Locked    36    169    0
30    Locked    37    179    0
31    Locked    37    205    0
3.1 Downstream channels

Channel    Channel Width (MHz)    FFT Type    Number of Active Subcarriers    Modulation (Active Profile)    First Active Subcarrier (Hz)
41    190    4K    3736    QAM 4096    148
3.1 Downstream channels

Channel ID    Locked Status    RxMER Data (dB)    PLC Power (dBmV)    Corrected errors (Active Profile)    Uncorrectable errors (Active Profile)
41    Locked    0    -14.6    432834    5640055

Thoughts? 

DD

And the errors seem to be increasing:

3.0 Downstream channels

Channel    Frequency (Hz)    Power (dBmV)    SNR (dB)    Modulation    Channel ID
1    194000000    7.9    39    QAM 256    8
2    138000000    10    39    QAM 256    1
3    146000000    9.5    38    QAM 256    2
4    154000000    9    38    QAM 256    3
5    162000000    8.6    39    QAM 256    4
6    170000000    8.4    39    QAM 256    5
7    178000000    7.9    39    QAM 256    6
8    186000000    7.9    39    QAM 256    7
9    202000000    7.4    40    QAM 256    9
10    210000000    7.1    40    QAM 256    10
11    218000000    7.2    39    QAM 256    11
12    226000000    6.6    39    QAM 256    12
13    234000000    5.9    40    QAM 256    13
14    314000000    3.5    40    QAM 256    23
15    322000000    3.4    39    QAM 256    24
16    330000000    3.2    39    QAM 256    25
17    338000000    2.7    39    QAM 256    26
18    346000000    2.8    39    QAM 256    27
19    354000000    2.7    39    QAM 256    28
20    362000000    2.1    38    QAM 256    29
21    370000000    1.7    38    QAM 256    30
22    378000000    1.5    37    QAM 256    31
23    386000000    0.9    37    QAM 256    32
24    394000000    0.5    39    QAM 256    33
25    402000000    0.3    39    QAM 256    34
26    410000000    0.1    39    QAM 256    35
27    418000000    0.5    39    QAM 256    36
28    522000000    -4.8    36    QAM 256    37
29    530000000    -4.8    36    QAM 256    38
30    538000000    -4.7    37    QAM 256    39
31    546000000    -4.7    37    QAM 256    40
3.0 Downstream channels

Channel    Locked Status    RxMER (dB)    Pre RS Errors    Post RS Errors
1    Locked    39    2399    1072
2    Locked    39    2202    2172
3    Locked    38    120    30
4    Locked    38    3114    2471
5    Locked    39    2622    2094
6    Locked    39    2459    1468
7    Locked    39    852    868
8    Locked    39    11    14
9    Locked    40    2414    1407
10    Locked    40    2270    1608
11    Locked    39    8063    2016
12    Locked    39    31738    3500
13    Locked    40    2177    1882
14    Locked    40    336    153
15    Locked    39    2118    1349
16    Locked    39    1845    907
17    Locked    39    2497    1435
18    Locked    39    2379    1477
19    Locked    39    27    23
20    Locked    38    28    0
21    Locked    38    131    1
22    Locked    37    66    0
23    Locked    37    47    0
24    Locked    39    92    0
25    Locked    39    3021    1950
26    Locked    39    126    4
27    Locked    39    3260    1142
28    Locked    36    436    0
29    Locked    36    3608    642
30    Locked    37    3402    570
31    Locked    37    4552    1142
3.1 Downstream channels

Channel    Channel Width (MHz)    FFT Type    Number of Active Subcarriers    Modulation (Active Profile)    First Active Subcarrier (Hz)
41    190    4K    3736    QAM 4096    148
3.1 Downstream channels

Channel ID    Locked Status    RxMER Data (dB)    PLC Power (dBmV)    Corrected errors (Active Profile)    Uncorrectable errors (Active Profile)
41    Locked    0    -14.9    1978520    18636098

Seems you have a faulty amp you are connected too.