This may seem an odd reason to be cheerful but I have had a few years to think about this and here’s my theory: When Ian and the Blockheads were writing the song back in Italy following the cancelled gig they were probably bouncing ideas off each other. The fourth reason to be cheerful they chose was the phrase “good golly Miss Molly” from the Little Richard song of the same name. It seems quite logical to me that “jump back in the alley” is another reference to Little Richard’s song, “Long Tall Sally”. In this case the phrase could mean listening to the song, or perhaps the fun that Uncle John is having in the original song!
Either way, they are both a good reason to be cheerful!