Yes, I thought about that too, but as far as I know there are no HF hams within a mile of the cable run.
There are clear RFI notches at the frequencies of the stronger AM stations in SW london.
Interestingly, while watching it some more, there have been several more complete dropouts along with dropouts of other spectrum chunks, all coinciding with throughput glitches.
The ISP is talktalk and I'm using a Fritzbox.
If someone has software (linux) which is better able to track the dropouts I'd be very interested. I very much doubt OpenRetch has any interest in hooking up spectrum analysers to monitor this kind of issue.
Do you want to stop spam, or do you just want to stop receiving it?