UK ISP NDO (NamesCo) is reporting that BT's own speed tester may be "broken", at least when dealing with speeds that are greater than 3Mbps. The news came out of a conference call between the provider and BTWholesale:

A few of you have noticed the apparent discrepancy between the BT speedtester results and the results from other speedtesters and I think this could be the cause. By the sound of it, it might take them a while to fix, so for the moment I would rely on the results from our speedtester as being the more reliable.

Secondly, there isn't currently a way for ISPs to "retrigger" the training period on a line. They are working on a solution to this, but it is proving tricky because if a lot of ISPs all triggered retrains at similar times it would have a hugely detrimental effect on the performance of BT's network.

It's worth pointing out that no speed tester, not even ours, can offer perfect accuracy. The remote server’s performance and other factors can also play a part. NDO's full post can be read HERE.

One of our mods has also run some tests on several ordinary website based testers and found that results can differ depending on your chosen website browser. We wouldn't be surprised if this was related to the caching system some employ.

Somewhat related - reliable sources have informed us that BT may issue a new update to its ADSLMax platform next week. We believe this could be designed to fix some of the speed issues.