I guess my Aunt and Uncle were more progressive in their naming habits than the Royal family...unless my cousins are actually John and Henry and no-one told me.
My cousins are called Jack and Harry, so since the early/mid 80's at least.
On a similar theme, why were John and Henry ever changed to Jack and Harry? They haven't shortened the name and have no resemblance to the original name. Likewise Why was William shortened to Bill and not Will, Edward...