I have only one socket in the flat
You mean you only have a master socket, to which the router is connected, and
no other extensions?
When you remove the faceplate of the master socket, are there any wires connected to the back of the faceplate? There should not be (as you have no extensions) and it should not make any diff whether router in test socket or in front socket. If there are wires on back of faceplate, there's your problem or you have a mysterious unknown extension.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 20 Meg WBC