Here I have shown the bench base before major work was done. 

The new timber supports can be seen as a lighter shade of timber, these are screwed to the normally fixed timbers on the frame. One to the rear inside, and one to the front inside.

When this has been done, "Piano" hinges are screwed to back of the rear fixed frame, and to the normally fixed timbers.

After the above is done the fixed timbers are then unscrewed.  

[On the near side front edge, due to wear markings on the wall, by pulling out the bed frame,  I have fitted stick on felt pads.]