Hi, welcome to the forums
First I'd just like to correct a misconception. It could be important.
You say the master socket is not one of the modern ones with the bottom half of the faceplate removable revealing a test socket. That being the case, plugging the router into that socket makes little or no difference in terms of eliminating the internal wiring. It's the existence of internal wiring connected to the master socket that matters most, not whether or not it is use.
I gather you are with Goscomb for your broadband, and that your phone line rental is still with BT. Is that right?
What did you report to BT the first time? Just a crackly line, or did you tell them about the broadband problems? If you did the latter that could account for you getting nowhere. What did you report the second time?
If you haven't already done this, I suggest you unplug your ADSL kit, any Sky boxes, and all phones but one. That one preferably being a corded not a cordless one. (You can get these cheap from several places and they are always useful for this sort of thing). Cordless phones and their power plugs can themselves be the cause of problems like this. I might even unplug them from the power in your situation.
Check what you think is the master socket really is. It has a capacitor as in the LJ 2/1A shown here
. The extensions are probably LJ 2/3As, seen next to it.
With the phone plugged into the master, dial 17070 and take Option 2. It should give you silence apart from the repeating message at intervals.
Have you looked to see if there is anything worrisome about the line as it goes through the trees? Such as it being forced badly out of line in any direction by growing branches? How about a pair of binoculars on it to see if there are any obvious worn areas in the outer?
My broadband basic info/help site - www.robertos.me.uk
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