so you had a good service, then what happened to prompt the tie pair mod ?
Zarjaz suspects there's a line fault
if you only have BTw in your exchange and it's small there may be very limited value in lift and shift unless it was a bad port.
A tie pair can be defective Your line would have a FTR based on its initial training, this is the fault threshold, so if the speed has halved you have a fault and should complain.
If you have audible noise problems complain to your voice provider.
In the end it's down to Openreach etc to fix via your retailers, we can't do it here
MaxDSL - goes as fast as it can and doesn't read the line checker first.