Here's the latest.
Whilst I was at work today, (ironically because I needed to upload huge files that were too slow to upload on ADSL2+) I got a call from Openreach on my mobile to say they are about to switch the line over to FTTC and if everything was okay to do so. Engineer said he'd do it and call me if there was any problems he could see, I had no call so I assumed everything worked correctly.
And it actually did! I'm now sync'd at 78224 downstream and 19999 upstream, which is really excellent. I was expecting less, but no doubt they made sure my line was high quality as possible after all the trouble I've had.
I just want to thank everyone on the forum, especially Black Sheep for your help on this situation, whether it's giving me escalation paths, information that helps me guide people to bark up the right tree as opposed to the wrong one or just suggestions that can help me try and kick people into action. I wouldn't have been able to do it without you guys.
Also thanks to my case manager, SH at Openreach. He has only been on the case for just under a week, but he's handled all communications within Openreach, given Sky a nudge when they need it and, got engineers out all day to fix this on Monday, personally made sure the order went through on Tuesday and kept a close eye on it and got the final bit of work done today. Also to S at Sky, who took over the case earlier this week and has been exceptional and very customer focused.
Have a good weekend all, and keep doing what you are!