The double is was a grammatical error popular in the 1970s and 1980s .
There are several other anachronisms , such as a police two-tone horn at a time when bells were still used , the skirt lengths and hairstyles of the schoolgirls , the fact that St John 's Smith Square was not opened as a concert hall until 1969 , the appearance of a 1975 Pentax K-Series camera , and at one point Jenny uses the `` double is '' , a grammatical error which caught on only in the 1970s and 1980s.