Hi B*cat. I'll try what you're suggesting within the next few days. One thing that has became clear is that although the interference is sort of predictable in terms of what times they appear on for the day of the week, it can't be timed to the exact minute on a predictable basis.
I'm 99% certain that the problem is not caused by something in my own premises. I occasionally indulge in AM radio DXing with indoor loops and I know that raised background noise levels are the bane of this hobby. There are two devices that I have which give off wideband hash interference, one is a SMPS that came with an Android "TV Box" and another is an old-ish Openbox S11 satellite receiver whose power supply looks dodgy anyway - I don't really use the former (I only need to get it set up again before giving it back to my brother) while the latter I only use now for helping align satellite dishes as I've now got a satellite receiver whose power supply behaves itself. It probably goes without saying, but in my experience a lot of cheap Chinese electronics & power supplies are poor at EMI suppression, if not outright dodgy at times.
When I was outside last Saturday when the noise was present on the line, I did have a portable radio which when I went to the telegraph pole my line is on where I held the radio next to a drop line that was in a duct before it goes underground - on that occasion the noise became stronger and the signal meter on the radio was registering it - of course that can just be noise from the ADSL service itself so I'll need to check between the noise being present and not being there to see a change I guess.
At the master socket I have a filter faceplace that I bought around four years ago - I think it was made by ADSL nation, but it's the same fascia as the filter on the right of the image below. I originally bought it when I was living in a flat and had a ADSL2+ Sky LLU, where it gave nearly a 1Mb downstream improvement over the filter supplied with the Sagem F@st2504n. Between the filter and the modem is a length of Cat5e cable - overkill perhaps, but it was readily available at the time. Blue twisted pair is used for POTS and green twisted pair for xDSL on the IDC connectors on the filter, which go to respective BT431 & RJ45 sockets. From the RJ45 socket is a short (30cm) CW1308 lead with RJ45 and RJ11 plugs on each end.
My local environment is in a small housing estate in a village in the rural Co. Tyrone countryside. My home doesn't have a farm attached to it(!) though there is a field less than 100 metres away where some newborn lambs and ewes are prancing around at present.
As I mentioned previously, there is a primary school just behind my back garden - though the time patterns of the interference would suggest that the offending interference is unlikely to come from there. There is a woodworks shop about 200 metres down the hill from myself, but again the timing of the interference would suggest it isn't from there. I can't think of anyone in the near vicinity that might use something like a welding torch. There isn't an electric substation right next to me, but it might be worth finding out where the nearest one is, I probably just haven't noticed it.
There are no armed forces camps in the local area (the nearest ones are about 35 miles away each, one a small RAF camp and another an Irish Army camp in Co. Donegal), the nearest radar installation - if it is currently in use - would be at an airport (St. Angelo) about 13 miles away. There are no broadcast transmitters in the local area, though I can see the Brougher Mountain TV & Radio transmitting station from an upstairs window. There is a 2G Vodafone mast approx 350 metres away in a straight line - the next nearest is a Vodafone 3G/4G & O2 2G/3G/4G mast at roughly 900 metres.
I'll report back in due course.
