It sounds like you have a BT NTE5 master socket without a filter built-in to the front plate and therefore are using the 'plug-in' filter that is included with the PlusNet router.
If this is the case then you could fit a socket doubler to the BT master socket and have your extension cable in one socket and a plug-in filter in the other socket. It is however also possible to remove the lower half of the master socket faceplate and connect the cable to the internal connection, this is the 'proper' way to add an extension socket.
See the pictures on this page - https://www.telecomgreen.co.uk/home-phone-repair/your-master-socket-and-how-to-avoid-a-large-bill/
This page is not specifically about adding additional sockets but it does show a good picture related to this. You can see the lower half front panel hanging off and on the back of it is where you attach the extension socket cable to. The whole point of this approach is that if asked by BT to do a test without anything else connected including the extension wiring you can simply unplug this front panel and all your own cabling is automatically disconnected. To do the typical BT test you merely plugin to the previously hidden test socket you can see was hidden behind the front panel.
This same site has other good articles like adding a second master socket, using the new NTE5c socket and so on.
In my own case I wired a second new style NTE5c with the built-in VDSL filter as an extension to the original BT NTE5 master socket to allow me to move my router. This was as per that sites other article - https://www.telecomgreen.co.uk/engineering/move-bt-master-socket-without-actually-moving-advice-tele...
(The original BT Master Socket is the 'property' of OpenReach and you are not supposed to modify or move it.)
