That first photo of yours detailing the right angled metal bracket with the bolt (that the hooked cam grabs hold of when the lock if activated) is the piece that I need. 

Does anyone know where I can buy one of these right angled brackets (as per Duckman's pic) as I cannot locate a part number anywhere and despite an exhaustive search online, I just cannot locate any reference to this piece.

Thanks

Not sure myself mate, cos of the age of the machine the hardware are a little harder to come by.  Unless someone is wrecking a Bally of that era i am not sure where you could source this part.