the hitchhikers guide to the galaxy - In the radio series, how did Zaphod get into the artificial universe?

In the book The Restaurant at the End of the Universe, it's very clearly explained how Zaphod gets into the simulated universe; he's in it from the moment he enters Zarniwop's office, and a big deal is made of him leaving via the window rather than the door so that he stays in it.

In the radio series, however, this doesn't happen. At the end of Fit the Seventh, Zaphod asks "How do we get to this universe, then?" and Roosta replies "I'm afraid it looks like we don't...", pointing out that the building is splitting down the middle, preventing them reaching the office.

Later, in Fit the Twelfth, Zaphod finally meets Zarniwoop in the lounge of the slightly-delayed spaceliner and reacts with astonishment at learning he's been in the artificial universe all this time. "How did we get into it? I mean like where, when?"

Zarniwoop only replies enigmatically: "You didn't notice? Well, I'll let you work it out for yourself."

So, how did he get in? As noted, he never actually entered Zarniwoop's office. My only thought is that it happened during the "body debit" incident with the disco, but there's no particular indication that that should be the case. Is there any other possibility?

Answer

Ahh but that is the whole point. You, the listener, aren't meant to know.

There are any number of possibilities, pretty much any loss of conciousness or teleportation of our heroes.

These can include your suggestion of the body debit card en-route to the Frogstar.

The exploding hyperspace field generator on Magrathea, shot by Bang Bang and Shooty.
The dismodulating anti-phase stun ray used by the white mice on Magrathea.

To name but three.

Don't forget that the script for the radio series was written during recording. The notes in the published scripts state that this was particularly true towards the last episodes. For example, Jonathan Pryce was hired to play The Man In The Shack but, as no script for the man had been written on the day he turned up to recording, he played Zarniwoop instead.

There are several places in the series as a whole where a convenient one-liner from a character recovers from a huge developing plot hole. It is usually a mistake to try to read too much into throwaway lines like this.