Can anyone help with a shower problem? (Non-EFC)

Not that kind of shower you dirty bugger, a normal shower. The top handle on my concealed shower mixer has come off and I can’t work out how the parts attach back together. I’ve got three loose bits: the square chrome handle, the chrome peg/barrel, and a small threaded pointed screw/insert. Photos attached. The lower two handles are still in place, so I can see what it should look like, but I can’t figure out what screws into what. Does the small threaded bit go into the peg first, then into the square handle? Or is there a missing grub screw/part? Also, does the square handle just push back onto the brass spindle on the wall, or is it fixed by the centre screw? On the bit in the concealed valve, there is like a small channel / groove that the screw must go into on some level, but it wont attach and when I screw it in it just goes round and round. Can you tell I have no idea what I am doing at all? Any help would be appreciated, if anyone is handy with this sort of thing! Cheers. PS: this was super easy to post, the new forum really is brilliant!