Just to agree with Partial, the photo you link to would imply two separate lines coming in on the dropwire at some point.
Could it be that the two sockets after are just linked in series, just that one is an LJU rather than an NTE ?
It may well be that as per your original suggestion, it's just a long line, and fitting a modern NTE with a faceplate filter will help. You really need to sort Edward Scissorhands work in the loft, if not a problem now, it will be soon enough.