Upload Speed has dropped

[#1]

Wonder if anyone can help or comment on this issue.
I had my FTTC service activated late December. I went for the Plusnet Extra Fibre with static IP.
Initially I was not getting download or upload speeds as advertised. BT Engineer was called in and he identified issues with internal wiring.
After fixing wiring, the BT guy did some tests with the rather expensive looking gadget he had and reported excellent sync speeds of 78Mbps. He said the line was very clear and couldn't identify any further issues. I had to call up Plusnet support to ask to them 'reset my sync speed' (if that's the terminology) because I was still getting slow speeds despite the BT guy having fixed the wiring issues.
The support guy at Plusnet waited on the phone with me for about 5 mins whilst he reset the line and after that I connected up and speeds were great. I got about 60-65Mbps down and about 16-18Mbps up. The 'Current Line Speed' (shown on the Plusnet member area under "High Speed Broadband") showed 78Mbps.
All good.
Everything has been working great for several weeks and I've had no issues with speed. However, recently I have noticed that my upload speed has dropped significantly.
Tests on Speedtest.net now show upload as 11-13Mbps and on the BT speedtester, upload comes through as 8-9Mbps. Not sure which one to believe.
Download speeds are still good, sitting at around 58-60Mbps.
My 'Current Line Speed' now shows 65Mbps, so this has dropped from 78Mbps which is what I had early January.
I've raised a ticket with Plusnet support, but keep getting told there is nothing wrong with the line. This is a bit annoying because there has definitely been a drop in upload in the past couple of weeks.
I do my tests at different times of the day to account for congestion, but they're all largely similar. I also do all my tests via ethernet, no wireless.
I have also done a speed test by directly connecting to the BT modem using ethernet. I get the same sort of speeds, with maybe 1 Mbps increase on the download and about 1/2Mbps increase on the upload. This is probably just overhead on the wireless router.
I've tried explaining all of this in the ticket with Plusnet but they seem reluctant to agree there is an issue and keep saying there is nothing wrong with the line. I disagree though, as there is a very noticeable drop in upload.
Can anyone comment or make any suggestion on what I can do to try and resolve this issue?
Thanks!
Phil

[#2]

Hi there,
I've just taken a look at this for you. I can see that whilst we've investigated the issue your upload speeds have varied between around 13mb and 16mb, without wanting to disappoint I do agree with the comments made by my colleagues that speeds can vary to such a degree. I can howver see that you've mentioned on the fault ticket an upload speed of around 8mb. AFAIK that would be significant for us to investigate further. When did you experience that as that isn't reflected on the speed test results I can see from the BT speed tester?