Ok , so the setup is pretty straightforward. One filter plugged directly into the master socket and one plugged into the socket in the study. A phone and is plugged into the filter in the lounge and one in the study along with the bband cable. I am sure you will have already had it setup in this manner, so that leaves us with a few more choices.
The long extension lead you used to test the broadband using the master socket should have been plugged directly into the master socket with no filter at that end the filter should then have been plugged in at the other end of the extension lead and the broadband line directly into that filter. Can you confirm that this is exactly how you set it up?
The line from the master to the study socket may also be faulty as could either or both of the filters.
Speedtouch modems are not the best, but it clearly isnt the issue in this case , so I think we can dismiss that as a cause.
The long extension lead you used to test the broadband using the master socket should have been plugged directly into the master socket with no filter at that end the filter should then have been plugged in at the other end of the extension lead and the broadband line directly into that filter. Can you confirm that this is exactly how you set it up?
The line from the master to the study socket may also be faulty as could either or both of the filters.
Speedtouch modems are not the best, but it clearly isnt the issue in this case , so I think we can dismiss that as a cause.