Each side of the back is generally slid down on to a spike on the metal action in the base and then locked down with a sprung clip which pushes a prong through the spike. A screwdriver can be pushed through the clip sideways to hold it open which frees you to use all of your might to lift the back. The back may be stuck on its spike so will require perseverance (and swearing).
Richard Oldroyd, October 2014
