After I joined BT last July, I took about 3-4 months before all problems with tel, broadband etc were sorted
The promised min 50 Mbps speed was never achieved, but in the end 40 Mbps was the max I ever got. However the last couple of weeks speeds are continuously under 20mbps which causes problems streaming movies etc.
If you have a Homehub 5 can you also post the stats from 1-12 by logging onto the homehub management pages then troubleshooting > helpdesk. http://bthomehub.home/
The stats for the Home Hub 6 can be found on the Information page from http://192.168.1.254 then Advanced Settings -> Technical Log.
Can you use this speed tester then carry out the further diagnostics and post back a screen shot of all the results including your IP profile for up and down. This test must be done with a wired connection.
Have you tried the quiet line test dial 17070 option 2 you should hear no noise if you can hear noise then it needs reporting to BT faults on 151 with no mention of broadband in the call are you connected to the test socket using a new filter