Yes, I did disable QoS. The odd disparity between the ADSL line stats and the speeds reported by online speed checkers (and the speeds we seem to get in practice, judging from the buffering that goes on with things like YouTube video and iPlayer) remains a mystery.
I'm new to getting stats from the system, and have only just started at looking how to do it properly, so, in a few days time, perhaps, I should have a better ideas as to what is going on.
The most noticeable change I noticed after doing all the work today is that the characteristic of all the download tests I tried had changed, markedly. Previously, I used to get a short period during the test at around 7 to 8 Mb, then a dip to zero, followed by a variable 2 to 3 Mb for the rest of the DL test. Today, every test I tried showed a steady figure of around 4Mb, peaking to 5Mb, with none of the drop outs that I've always seen before.
Whether these drop outs were caused by either of the two modem/routers I had been using (for the past year I've been using a TP-Link TD-8817, but switched to the PhoneCoop provided Technicolor TG582N last week, to see if that made an improvement) I don't know. I do know that the HG612 wouldn't work as a modem and router, no matter what I tried, but seems very solid when used as just a modem.
Hopefully I'll have some time to gather some stats properly before we get switched to VDSL on the 14th November, as I'm curious as to why there should be such a disparity between the reported line stats and reality. I'm also curious as to why the consistent initially fairly fast (close to the max the line would support) speed would drop right off after a short time when I was using either of the other two modem/routers.