After scanning the wares at gelatomassi twice in one week, and not spying any "peanut butter" gelato, tonight I asked for it before making my decision. They had some out the back. Score!! I guess sometimes you do have to be a bit of a pushy customer, and ask directly.
It seems Dolce and Gelato have also joined the craze for peanut butter flavoured items (I spotted some last week).