Just went out to the shed for another look, i don't think it has tried to be set to freeplay.  From what i understand, the credit zero position switch (blue arrow in picture on previous page) needs to be set to permanently closed to set to freeplay.  The top two blades of this switch have all original wiring on them - in fact all of the wiring in the back box looks at least 35 years old!!.  Unless there is another way to set it to free play?