Thanks for that link, the BT speedtester reports my profile OK but that one is easier and (I assume?) not subject to the 1 per hour limitation... bookmarked
I found the tbb tester to be consistent as well, but it was consistently wrong
I use a Mac, which has a little monitor showing network traffic. On several occasions I ran three consecutive tests- speedtest.net, tbb and BT. All three showed a nice flat-topped curve so no peak/average problems, but the tbb one was consistently only about 60-70% of the height of the other two.
I can only think it was peering or routing oddities, but I never got to the bottom of it and it appears to have gone away when I went to the 80/20 connection