BT Broadband - capped at 2 Mbps - who can help?

I upgraded to BT BB from BT yahoo BB 3 months ago, hoping to get 8 Mbps.

I'm only getting 2 Mbps (including on the BT speedtester using the
speedtest login).

From day 1 (actually about day 20 - because BT 'forgot' to activate
ADSLMax for my line) my modem has synced at 8128. It has never synced at
any other speed, nor dropped a connection.

My DS SNR margin is 15 and attenuation 11. My modem logs about 1 CRC
error per 24 hours.

I understand that the most likely cause of this is that the BRAS profile
on my line has got stuck at 2000 kbps.

Unfortunately, BT BB tech support are unable to help:
I've tried helpful phrases like:

"There were some problems activating my line. My throughput is only 2000
kbps even though my sync speed is 8128. Could the rate limit have been
left at 2000?". When this didn't work, I tried a more direct approach.

"My modem always syncs at 8128, but my throughput is never more than
2000. Can you check the BRAS profile, please?".

The reply to this was to ask me if the green lights on my router were
all illuminated. When I duly explained that they were, she went on to
ask me to delete the cookies in internet explorer, despite my
protestations that I don't use internet explorer. In the end, I had to
humour her, just to get to the next stage.

So, 10 phone calls to tech support. On 3 occasions, I've had my query
passed on to the engineering department - but have not yet had a reply.

Is there a 2nd tier support desk that actually knows something about
their broadband service? I'm getting very tired and very frustrated
(made worse by the fact that BT have been double billing me for this
fiasco).

Yours patiently,

Frustrated from South Yorkshire.