OK. My question is. Is it possible to remove the magazine whilst holding the swivels? Not sure that I fully understand what part of rifle (18) they held when they removed it from Sheila. 
You don't need to hold the gun to remove the magazine. With it sitting down you can remove it. You just need to push the release button and pull the magazine out.
My understanding is that the magazine was removed at the lab. No separate reference was created for the magazine early on. Hammersley didn't remove the magazine on the day of the murders it was conveyed while still in the gun.
That is why I was asking who you were referring to as to removing it. It obviously had to be removed to load and test fire the weapon. I am not aware of other times it was removed though.
It didn't need to be removed by the firearms officer because when the weapon is empty the bolt remains open. Woodcock was able to look inside the weapon to see the chamber was empty and that the magazine was empty because with the bolt open you can look into the chamber and the top of the magazine is right there.
Here is it with the bolt open:

This is it with the bolt closed:
The bolt only stays open when the gun fires the last bullet in the magazine.
That is why the magazine wasn't removed to "make the weapon safe". Nor is there any evidence it was removed to be a separate exhibit at the time of collection so it wasn't removed till later during forensic testing according to the police involved in the collection.