They keep asking you to run speedtest aswell when you've changed your filter/ tested at the master/ they've run there tests etc and if they knew the speedtester.bt.com they'd know that you can only run 1 test per hour.
So i tested mine before i rang them so i have my results ready and then there asking me to run one again? yeah if you wanna wait an hour mate.
These guys are running from a crib sheet. They aren't really interested in your answers, they just go on to the next question
If you say to them "I don't need microfilters because my broadband is isolated from the house wiring at the master socket, please escalate my complaint" and repeat that again and again they will eventually pass your complaint back to someone in the UK that can deal with it
And when the guy with the laptop arrives at your house he will be able to tell, within 5 metres ,on a line length of 4km, where the line fault is, or if the exchange equipment is rusty or past it. They are generally that good
However, if you have not isolated your house wiring and the fault does turn out to be indoors, they will want to charge. And why not?