TBB speed tester slower than others?

Been noticing that the thinkbroadband speed tester is giving significantly slower results on my connection than other speed-test websites:

offpeak TBB gives 300-400kbps
peak hours TBB gives 40-50kbps

offpeak speedtest.bt.com gives 1700kbps
peak speedtest.bt.com gives about the same

you can substitute the bt service for pretty much any other you find on google and they all come up about the same.

Is there something different about the TBB Speed test?
Is it indicating a potential problem somewhere?
Am I likely to get a sensible response to this being raised as an issue with my ISP?

I'm not saying my broadband is fantastic, it's pretty choppy for browsing, but have been spoilt in the past with 10-12meg D/L (actual) at a previous address..

might be worth tracerouting to the various testers to see if any obvious differences occur.

tried a traceroute to 2 of the testers, TBB and broadbandspeedchecker.co.uk.

Interestingly the route to the TBB tester was done in 9 hops and each was below 100ms and on average below 50. The broadbandspeedchecker test server, hosted by amazon was hosted in the US and the hop time increased significantly once it had crossed the pond, and timed out on hop 15..

I'm struggling to find the actual test servers used by some of the other more popular test sites.

The O2 support guy gave me a url for be* server, I must find the paper that was written on...

G