This is what I have in my attic.
So the black cable comes in from an outside pole. It reaches this junction box then splits into two extension sockets. One is unused and the other is in my bedroom.
(1) could this setup be the reason I am getting lower speed than I should (10 to 20 mbps from an infinity 2 connection and phone line rated at 40 mbps)
(2) If I asked for a BT engineer to come would he fix this? would i get charged
Just simply disconnect the wires that go to the unused extension as stated previously, the 2 cables are different colour sheathing so should be obvious which one goes where.