ADSL line mode is now T1.413 and attenuation/noise changed

I thought the UK standard was G.dmt but for the last 2 days my router ( BT
Voyager 2000) is saying that the connection is T1.413. I can force it back
to G.dmt but if I then do a subsequent re-connection it connects as T1.413
again.

Over the same period, the line attenuation and noise margin figures have
both dropped too: attenuation from 50.5 to 26.5 and noise margin from 23.3
to 13.

To complicate matters more, these lower figures are what I used to get
before I improved the extension wiring in the house by taking a dedicated
feed from the BT master socket for adsl. I made the wiring changes because
the adsl would typically lose sync several times a day; after the changes it
was generally fine. So, despite the much worse attenuation figure (but
better noise margin) I had a more stable connection.

I haven't changed any wiring recently so now I'm confused.

Any ideas what's going on?

Thanks.