Following on from MrSaffron's post, the IP Profile is what determines your maximum throughput speed. It adjusts to a figure determined by the connection speed, but lower. When you get a low connection speed for whatever reason it drops almost immediately, but it can take days to rise when you get the decent connection speed back.
, and a table of IP Profile speeds.
You can find your IP Profile by running the BT Wholesale Performance Test
, (ignore the red instructions for now, just say you've done them), and on the initial results page click Further Diagnostics
near the bottom.
It's worth this time copying and pasting the full contents of the down and up text boxes of results. We don't need to see the graphics.