Pretty sure we tried going straight in the test socket with an external filter months ago, and it was exactly the same.
The same speed-wise? Or stability-wise?
Assuming it was the same stability-wise (ie bad), this problem looks to require attention from an Openreach engineer ... so it will be worth finding out who attended before.
If it was an Openreach engineer, you would expect him to have run a variety of tests, both run remotely (started from his mobile) and from a handheld test unit ... which may have led to a physical fault that needed fixing ... which, in turn, could have led to a DLM reset being performed. The latter is, as far as I'm aware, the only "settings" that an Openreach engineer would attempt to make changes to.
Currently, a DLM reset could go one of two ways - one to an "open" profile without interleaving, and the other to an interleaved profile ... and this might explain why you now see interleaving present. But I would also have expected a DLM reset to have lifted the banding limitation ... so it isn't immediately obvious what has happened there.
Meanwhile, a "bright sparks" engineer might have played with the router somewhat, and "changed settings" there ... which would likely have no effect whatsoever on this fault. A result of "changed some settings" makes this one sound more likely ... but shouldn't have caused interleaving to be added.
Do you have any statistics from before the visit?
I'm not very au-fait with the TalkTalk router, so others will have to advise whether more information can be extracted to help diagnose.