Thank you for the nice comments.
Chooksta, I am tempted to open the locket as there could be something good inside though it doesn't seen overly heavy so I doubt it. I am certain to break the hinge. Incidentally I googled mizpah and wikipedia came up with the following:
Mizpah is an emotional bond between people who are separated (either physically or by death). Mizpah jewelry is worn to signify this bond, and the word "mizpah" can often be found on headstones in cemeteries and on other memorials. From Genesis 31:49 of the Bible
"And Mizpah; for he said, The Lord watch between me and thee, when we are absent one from another."
Given the age of the coins found with it I'd suggest it was given when a loved one left Britain for Australia.