Ongoing massive speed drops

[#1]

I'm very disappointed that neither my comment to the ongoing  fault report this time yesterday, nor my comment elsewhere on the forum, have been acknowledged.
The original fault report took a week to be picked! It was actioned by an Openreach engineer attending last Saturday (and following up on Monday). But despite all his efforts I'm still seeing a massive drop in FTTC speed in the evening.
At 1500 my (slowest over four tests) download/upload speeds were 50/18Mbps ... 1700 they were 40/16 ... 1730 down to 20/11 ... 1800: 13/11 ... 1945: 6/8  ... 2100: 4/17
The most recent comments from PlusNet are on Friday 5.28pm putting the question on hold until Saturday ... it was taken off hold 7.05am on Saturday ... and I followed it up on Monday at 9pm pointing out that the engineer's visits did not fix the problem.
Is somebody at PlusNet going to get a hold of this? I really don't want to have to go through the weeks and weeks of getting this sorted that I had to last time this happened (albeit on ADSL) when the actual problem was the obvious one (exchange congestion).

[#2]

It looks like the faults guys have done some thorough investigation already. It's worth them double checking the capacity issue though as it's unclear if this has done yet. 

[#3]

If they've done thorough checking then they'll have looked into the capacity issue I guess. But nothing has been reported back since last week. Also, capacity checks don't always seem to tell the truth (as proven with my problem with ADSL when PlusNet told me everything was green, but after a long investigation, a second Openreach engineer discovered a "hot VP" which was not being reported).
The daytime speed has settled nicely to about 60Mbps (which is close to my IPprofile), but the evening speed is still abysmal (currently 4Mbps and pretty much unusable).
I'm still looking for a solution.

[#4]

Thanks - now have a second engineer visit on Wednesday.
What I now need is to find a way of convincing him that my slowdowns are congestion-related, and he doesn't go away and spend ages repeating what the first engineer spent 7 hours doing, particularly as he'll be here when the speed will be excellent.
Today, because it's a holiday it's running slow all day. Other friends/neighbours are having similar problems but I haven't been able to persuade them to report it as a fault because "it's way too much hassle" or "they just blame my equipment so what's the point".
Ho hum