My built in laptop microphone doesn't work so I got an external one to plug into the 3.5mm headset socket.

The microphone works fine, though I cannot hear playback through the built in speakers (probably because the laptop thinks a headset is connected rather than just a mic)

Please can someone help me channel the output to speakers. When I try to do this through pulse audio settings, the speakers are showing as "unavailable)

Don't know which Plasma version you are using, but normally the output can be set by Application. Klick on the speaker icon in the System Tray.

Any reason why the built-in microphone doesn't work? If this is an issue with your sound card, you might have other problems linked to it, sometimes getting another sound card is the solution.