I spotted your post in ISP Unhappiness; rather than cross-post I thought it best to reply here.
I'd recommend running the BT speed test at the time you experience slow speeds, so we then have a record we can present to BT showing when the problems occurs - from what I can see last time you contacted us the BT speed test result was good. However some tests showing poor performance should help add weight to a fault report.
If you're still in contact with others having the same problems then suggest they do the same with their ISPs.
ZeN Performance and Process Improvement Manager