I'm pretty sure it's not my end (iMac, quad core i5, Airport Express, LAN is gigabit), I never saw it on ADSL2+ either, IDNet guarantee no congestion on their network and don't throttle anything, and if it's not the tbb end that only leaves the bit in the middle- BT 
Similar to my setup, Dell Core i7 PC, Win 7 Pro x64, Gigabit connected to port on Draytek 2820. No congestion here on BE.
Re your reply to Sylcol about tbb's Flash speedtester- it's slower than the Java one for me 
eta- Congestion... at 3am?
I doubt congestion, especially at 3am. Still makes no sense.
On my link right now:
Flash = 14.86 Mbps download, 1.02 Mbps upload
Java = 14.5 Mbps download, 1.1 Mbps upload
So pretty much the same, given rounding differences at the 2nd decimal.
--
James -
be* pro, on THFB exchange with a Draytek 2820Vn and a BeBox (585v7)
BQM