In America, We Say A Show 'Jumped the Shark'

This comes from a '70s episode of a program called Happy Days (1974-1984), with Ron Howard and Henry Winkler as youths in 1950s America. With the gradual evolution over the years, the show failed to maintain the 50s fasions and Winkler became too much of a superhero type.
In one episode, after the movie Jaws, Winkler jumps over a shark on a dare, all the while wearing his black leather jacket. It is considered the lowest point of the show and with cast changes that followed, the consensus is Happy Days was never the same again.

So now when a show that was once good goes bad, we say it jumped the shark.

This idea was revisited when Tom Cruise appeared on Oprah Winfrey's talk show to announce his marriage a few years back, which has clearly since ended in divorce, and he stood upon her couch. This was now known as jumping the couch, a low point for an actor's career.

But back to jumping the shark.

When would we say On The Buses jumped the shark? I've noticed with the sixth season, there is a strange change in the character behavior, and that Jack and Arthur will be leaving the show.

What I'm watching now, that burning bus in the first episode of season six, No Smoke Without Fire, seems to be a pinnacle. I liked that episode, but it seems they didn't quite know how to top it. I'm at Bye Bye Blakey, and this is all but an American-styled episode.

What's considered the event that made On The Buses lose its charm, or maybe it never did?