In reply to Mountain Llama:
So I managed to unscrew and loosen the simple locking system but now that part has gone further up the tube and it's harder to reach to take out. I was trying to remove the grip of the pole so that I could fit something thinner and long from the other side and push the broken part through the other side but I don't think there is any way to remove the grip without breaking something... :/
Also I noticed that the system is not like the one I showed in the photo, but rather a split dowel, which means the screw is a very specific plastic pin which has a part in the shape of a cone that is getting bigger so the more it is screwed the more the split dowel expands.
I can find lots of replacement split dowels but I can't find any replacement screws/pins
Is there any place where I can get this replacement part? Or is there any way I can buy something like an attachable flicklock that will do the same job but from the outside of the tube?