Well, VF have bucked their trend and been really good!
Got a call today saying there was definitely an issue and they'll send Openreach round to reset the DLM. He caught on that I was interested in this stuff and we had a good chat. Again, seemed perfectly cool with me using the HG612 modem instead of theirs. So, OR engineer coming round on Thursday! In the day-and-a-bit I had the VF box connected, it dropped its VDSL connection twice. My line definitely prefers the 612!
In the meantime, I gave the customer side of the NTE5 a little spruce. I hadn't been using the phone socket - there's just an extension running to another room using the terminals on the back of the faceplate. I disconnected and cut back the bell wire, leaving only 2 and 5 and put some contact cleaner on all the BT plug/socket parts.
My DSL cable was about 3 times longer than it needed to be, so I lopped the slack off and punched it into the unfiltered terminals on the I-plate. Interestingly, the freebie cable doesn't appear to be twisted-pair -- just straight-through.
So, the HG612's back in use and reporting very slightly better attainable than before. The ADSLNation cable's on its way so it'll be interesting to see what that achieves -- purely an academic interest, I'm not expecting a significant increase!
Thanks for all your advice!
Chris