The thing is that we're in a very quiet semi-rural area.
There is nothing between us and the cabinet that immediately pops out as being a source of interference. There is a pub but since the interference appears to be there all the time, not just when the pub is open, would seem to indicate that that isn't the cause.
The fact that previous fault tests have picked up a seemingly intermittent fault that magically goes away would seem to indicate an issue of some kind. The telephone line has been crackly in the past (I didn't actually know about this but other family members have indicated this) so this would seem to indicate some external issue.
I remain puzzled by the missing upstream tones because as has been said by others, the graph is looking quite healthy when suddenly all the tones just vanish.