Hi guys i have done some tests with all firmwares and the 2850 with adsl
I have find out that some having bad lines or very long, the G.DTM may be helpful to turn it on but expect 1-3 mb less synchronizing so may worth trying and a firmware with longer loop attenuator line because seems more stable at least to my tests (my provider is TT not BE but still may get similar results)
Maybe testing a firmware with lower line attenuator will help better??(for bad lines)

Have a look at my feedback and which firmwares have lower/longer attenuator and leave your own experience here
http://www.forum.draytek.co.uk/viewtopic.php?f=12&t=17573

Just want to post a quick update to my previous posts:

I've reconnected to ADSL with the Draytek Vigor 2850.
There has been a bit of a delay in reconnecting the Draytek as I wanted to do some comparison results with varying lengths of ADSL cable (0.5m, 2m, 5m, 10m) as well as with a new filtered master faceplate in place (ADSLNation XTE-2005), so I didn't want to remove the Thomson TG585v7 while I did this. (For the record, differences have been fairly negligible and I haven't made any tables of results, so please don't ask).

With the Draytek in place, Sync speed has dropped to just over 10Mbps, Speedtest results hovering around 9.5 Mbps.
There's a bit of a drop in Sync Speed compared to the Thomson, but the connection has only been up for 12 hours, albeit solidly for 12 hours, without any router reboots. No disconnections whatsoever. Basically the same SNR of 6 as compared with Thomson.

Unfortunately, I can't monitor with RouterStats to compare results with the Thomson but I'll look into what I can do with logging in the near future. My main priority now though is to sort out the rest of my network setup.