@Tubaman, @Alex Atkin, thank you both for your really helpful responses.
I can share you a DSLstats screenshot highlighting SNR stability. It follow the pattern I have come to expect from reading this forum; improving in the day, weaker at night, and a slower trend which mirrors the weather conditions (cold & dry is good, warm and wet is worse). The step at the end is the resync from this morning. The second attachment is the potential sync speed (and actual sync) shown by the HG612. I'm aware it is reported to be a bit optimistic but yes, it does clearly show the banding. I am no expect but both look pretty stable to me, much better than the 8800NL R2 I had on there before.
Question - in the "Additional diagnostic" section of the BTW performance tester at
https://speedtest.btwholesale.com/ it tells me the following:
IP Profile for your line is 10 Mbps
. This is what would expect, and does reflect the limit Fibre 1 package we are subscribed to.
Despite this I do see a 2Mb/sec limit in real performance, as per the screenshot in my original post.
For full disclosure I should say that the BTW performance tester will report around 6Mb / sec real performance, but that seems to be because it takes the peak upload speed as the measurement, and not the stable upload speed. I have attached a screengrab from speedtest.net which demonstrates this - a relatively high "burst" rate followed by a very consistent 2Mb / sec rate (upload is in purple). You can see this effect on the BTW speed tester, but only as the numbers flash past your eyes, before it reports the peak number.
I have seen this burst-and-stable pattern before in speed limiters, which is what makes me thing there is some limiter in effect here, separate from the raw syncs of DSL.
My conclusion is that I'm being limited at an IP / PPPoE level and not at the raw DSL level, but I don't know enough of the architecture of how my device actually appears "on the internet" to know where my traffic goes to once it gets into the cabinet and therefore how / where this would be done.