Thanks for the further input, but I think there's no more to be got from this line. It was very good on ADSLMax, but it's CARP for the current ADSL2+ product. Whether that is caused by the line or the ADSL2+ product I wouldn't like to hazard a guess. But with all this talk of DLM (that dark mysterious Black Box) that seems to have limitless powers to screw up an end users broadband experience, probably the best I can do is to stop using that line and stick with the O2 LLU line instead.
@coolsnakeman - yes, of course the router is compatible with ADSL2+, it's a Netgear 834G v4, and I have it set to adsl2+ under Dsl Mode, under ADSL Settings. I'd have to be a bit of a numpty to start this thread if I hadn't done most of the obvious things and then ask for peoples views on this situation, wouldn't I ? As for the test socket, been there, done that, seen the video and I've got the t-shirt in all the available colours.
@rizla - I'm inclined to agree with you about the upstream rate, my O2 Upstream rate is currently at 1256kbps, but I think the default upstream rate on Plusnet is 888. You can request them to crank it up a bit, but as I said before I don't need more Upstream, but I could definitely do with a bit more on the Downstream. It seems to me that this just ain't gonna happen - I have a line which is not performing as I was told it was going to (even after tidying up my internal wiring), so I feel totally justified in cancelling the Plusnet services. I suppose I could ask them to put line into training again, and see what happens, but I think I'd just be delaying the inevitable.