Got to love freek results from speed tests - BT's

[#1]

Upload speed achieved during the test was - 4.75Mbps
Additional Information:
Upstream Rate IP profile on your line is - 0.83 Mbps
On a DSL line Not weetabix

[#2]

I take it you've not seen that on further speed tests and was maybe a one-off?

[#3]

Nope a one off, I normally use BTs and TBBs tester and if neither match, I use speedtester.net.  Then I got a solid 0.7mB/s from TBB and speedtester after getting that one from BT.