I just bought a mixer (https://www.amazon.co.uk/gp/product/B000KGYAYQ/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1) and it does technically do what I want. It puts multiple inputs into 1 output however the issue I have is that it only gives sound in one ear of my headset and the sound is really muffled unless I have it really quiet. I fear this is because I got a cheap mixer however it has got good reviews and people say it works really well so that doesn't seem right.
EDIT: I have got 3.5mm to 6.35mm adapters for the audio jack heads so it will fit so that may not help.
EDIT: I have got 3.5mm to 6.35mm adapters for the audio jack heads so it will fit so that may not help.