frudemoo

Yes, that's it exactly Grant! Thanks for the pics

I think I need to see how that large spring on the left makes contact with the metal... I was fiddling with that and noticing small differences in the movement. I think perhaps the little right-angle on the end of the spring is meant to sit somewhere specific? I can't quite see in your picture.
I also can't see any reason why the button would not be working, but there are so many little washers and things on the underside I can't work out what any of them do.