I still can't understand why the DLM would take interleaving off altogether
New connections are put on Interleaving by default.
If your line MTBE drops to ILQ green, then DLM removes interleaving (can't recall exactly how many err secs that converts to but its on the main site).
at the same time increase my speed to 39mb+
As mentioned in previous posts - interleaving and error correction carries overheads, these cost you some line speed. When interleaving is removed most lines will see a decent increase in their sync speed.
& drop the SNR 4.8
That is nothing to do with DLM. The ECI cabs have a target SNRM of 6.3 dB and nothing less than that - only the Huawei cabs have 'x'dB. Therefore the 4.8dB must be attributable to noise on the line or one of your crosstalkers coming on line.
I suspect your line sync'd during a good period, but as the interference kicked in, this will be what caused your SNRM to decrease.
The L & S was done when I was on copper
It's my understanding that the term lift can be used to cover various scenarios... but usually it is assigning you a new port at the exchange using the same copper run. Line pair change is the term for changing the copper. I've had a couple of what I'd deem line pair changes when I've had a fault in the past, yet I've seen on the report when it was swapped to a new pair at the UG JB right outside my house being reported back to the ISP as a L&S... so who knows what they really changed on yours unless you saw what they did.:/
Can anything be done?
They could try a pair swap and hope you get an understanding engineer. Your ammunition will be your DSLstats graphs showing the decreases and increases in SNRM. You need to have a few days worth of SNRM graphs to show him so he can see the varience. The varios Errors graphs will also be useful to show.
They flatly refuse to un-band me unless I agree to an engineer & accept the charges.
They 'should' be able to request a remote DLM reset at no cost to them. It's not an automated process so not all ISPs like doing them. As far as the engineer and charges, then that is a risk only you can decide on.
I took the risk, the engineer came out and did a pair change swap and wasn't charged..... BUT I was told I'd been put on the last available pair which was only in slightly better physical condition than the one they changed it from despite the fact it sync'd at about 2Mb lower.
I was told that as it was the final spare pair, then he wanted me to see how it ran for a couple of weeks and if the problem was still there, then Openreach would need to pull a whole new length of cable through which was something he couldn't do. Sounded reasonable so I said OK. Two weeks later I re-reported it and was told nothing more could be done and that I was above the min threshold. I feel I was a bit fobbed off by the attending engineer and have ended up with a line that syncs less than the old one... however this one does have slightly less errors when my daily error burst starts... so I guess as far as Openreach is concerned then it is the better line.
BlackSheep kindly checked the Openreach reports for me and couldn't see any mention of the pulling a new line to my premises, so perhaps I was fobbed off just to close the job, but he said he didn't think I'd get any further. :/