That's a horrible graph. SNRM can fluctuate by a few dB within a few hours, and a few more dB at night, but yours is all over the place. I hear what you say about a neighbour having similar issues but, all the same, I'd be inclined to try and eliminate and problems internal to your home.
You say you have a filtered master, but is there any extension wiring? If so, perhaps disconnect it, and see if the SNRM stabilises?
It could also be a faulty electrical appliance, or perhaps a thermostat in you CH, or even faulty house electrics. One way to narrow it down is to go to the consumer unit (*see below) and ask somebody to watch the SNRM while you switch off each circuit in turn, except the one that powers the PC & router. Then unplug everything else plugged into that last mains circuit, one at a time, with somebody watching the SNRM again. During all of that, if the SNRM suddenly stabilises or improves, you could be onto something...
Other possibilities would include things such things as faulty street lights, have you seen any patters (like time of day) when the SNRM is better or worse?
Old AM radios used to be quite susceptible to switching interference. If you have one, you could try wandering around the house (or streets) with a radio tuned between stations to see if it yields any clues. Unfortunately a certain amount of buzzing and popping on AM is quite normal, you just have to follow your hunches.
edit: * My suggestion is based on the more recent consumer units with an individual switchable trip for each circuit. Wouldn't be much help on an old-style fuse box.
