Slow Speeds since Install. Mods??

Question

Any Mod brave enough to look into this one ????
It's gotten to the point of me not trusting a thing Support say and they don't seem to listen to a word I say.
I was told before signing up "Infinity will never slow down"
"The speed we estimate for your line is the speed you will get"
Since having Infinity Installed 04/04/2011, my speeds during the day are good at 35mb,
but slows down every night, sometimes as low as 4mb.
I reported this to BB support,explaining clearly that the connection is fine during the day,
but seems to be suffering from congestion slowdowns in the evenings.
I've been on the phone constantly since opening another fault report on 7th May.
The first few days consisted of BB Support calling me back in the mornings (when the speed is fine),
telling me they changed something and it should be ok now,
followed by me phoning at night when the speed dropped again.
This was escalated a couple of times to the "engineering dept.", still no change.
I soon realised that if I said my speed was fine at the time they asked, nothing was being done.
11th May.
An engineer was booked for my home for the 12th 8am-1pm.
I tell them this was probably a waste of time as the problem only appears at night,
and the line is fine during the day.
12th May.
Engineer calls about noon,very friendly,I explain the problem and we have a nice chat,
line check was good 40/10, no surprise there then.
BB Support phoned me as soon as the engineer had booked off the job,
basically telling me the fault had been fixed as an engineer had been out and found no fault.
I argued that they couldn't say that as the slowdowns only happen at night.
I point out to them I've been using speedtester.bt and ask them to look at the results, they totally ignored this.
Back on phone that night. The vicious circle starts again, another couple of days of callbacks telling me they changed something, and me calling them at night.
15th May.
Escalated again,I'm told a higher level engineer would visit the exchange and then my home.
16th May.
The same friendly openreach engineer turns up at 9am, I just smile and tell him he knows where it is.
No faults on the line and he explains he has nothing to do with the exchange.
Callback from BB Support around 3pm asking if my speed was fine now.
I had just finished another speedtest (7.3mb) so, no it wasn't.
Another engineer appointment made for 17th
17th May.
No engineer. 2 callbacks, go over the same old story. Engineer booked for 18th.
18th May. No engineer. callback that night,by now I know the script by heart. Engineer booked for 19th.
19th May. No engineer. Another booking for 20th.
20th May Engineer calls, we have a chat, I explain the problem, engineer leaves.
By this time I feel sorry for these Openreach guys,sent out to jobs they can't possibly fix.
They could be out connecting other people to this superfast connection that I'm not getting.
Callback in the afternoon,was told the engineer had contacted BT Wholesale, but no further info was given. They asked how my speed was, I ask them to call tomorrow after I log the usual slowdowns with speedtester.

Answer 1

Thanks for your post,  but I've already been through all your points above.
I have taken continual speedtests since the install with speedtester.bt as well as others.
The really slow period is usually from around 8pm till 11pm.
The 3 results above are just from last night.
I keep records of everything
The problem is that nobody looks at the speedtester results.
If they did, they could see exactly when this was happening and maybe take the proper action,
and save me going over the same sad story when speaking to the Indian call centre.
link to my speedtest.net results page
http://www.speedtest.net/results.php?sh=8dafd1fe7cc1723ab932e1e25ec9d957&ria=0
thinkbroadband graph
Forgot to add
I asked BB Support to have my line monitored as this was happening during peak times,
they totally ignored this suggestion.