Hi Andrew,
If DLM has increased your target, as seems likely, then it is supposed to gradually reduce it again, in 3dB decrements. Each time that happens, your line will spontaneously resync at a 3dB lower SNR and higher speed so you just have to leave it alone.
But to qualify for a reduction, you need to convince DLM that your line is not just stable, but 'very stable', and that needs to be sustained for quite some time. Again nothing is publicly documented, but there's a lot of anecdotal evidence that 14 days of stability is the period it takes to satisfy DLM.
So I'd say the first thing to do is simply wait for about 3 weeks and see if it improves. I might also be inclined to resync your router (just once) late in the evening and then leave it alone. That way you'll probably get an even lower sync speed. But because your sync speed is lower, so your error rates should also be lower, making the line look more stable to DLM.
You can also plead with your ISP, and explain how you may have accidentally caused the target margin to get raised, as ISPs can ask BT to manually reset the target. Unfortunately, for reasons I've never seen explained, that sometimes seems to be only a temporary effect, with the target margin shooting back up to 15dB again a couple of days later.
BTW - you're not alone. I managed to get my own target increased just over a year ago, thanks to an afernoon spend 'experimenting' with different routers, sockets & filters. I waited over a year and DLM never fixed it, so don't hold your breath...
7LM
