Thanks for the photosShutterfinger and I can report some success! A carefully applied amount of bending of the small prong on the timer top plate and gentle leverage enabled me top un-jam the timer setting lever. I then retrieved and re-attached the cocking ring spring. Next step is to reattach the cam plate and see if things work.
After a bout of health problems I have finally got back to looking at this project. Having reasembled the faceplate etc I find that the shutter cocks but will not stay cocked, it runs as soon as I release the cocking lever. It isn't clear from the various drawings what exactly holds the shutter cocking ring in the cocked position so further investigation is required.