A&A have settled on blaming BT for passing a duff BRAS profile and have manually upped their shaping to 38M, I can now get a steady 4.5-4.6MiB/s on downloads. The TBB speed-tester says 35.8M but if that's based on 1M=2^20 rather than 1M=10^6 then it works out as 37.5Mb/s and speedtest.net says 37.2-37.5M.
The BT speed tester is consistently lower than the figures above at about 34000kb/s but it was 30000kb/s previously.
So I'm pretty happy with the outcome thanks to an ISP with decent technical support!
Ugh - that's a problem. I didn't realise there were any testers who did use 2^20 counts; I just assumed that I couldn't assume.
Doesn't it all feel just a bit too much hit-and-miss? The amount of time you have to explain why a line's speed is not what you'll ever see... 5% invisibly lost on FEC, 10% lost by ATM overhead (well, on ADSL), up to 11% can be lost because of the IPProfile thresholds, and another 1% on AAL5/PPP/IP/TCP headers. Add in another 5% variability for the 10^6 vs 2^20 counts.
So many people who think they are missing out on something... but can't really tell.
If you access Clueless, what does it tell you your BRAS rate is there? (usually on the 1st graph next to your telephone number).
If it states 35M then it has been capped, but give A&A a call and the will manually set it.