SINCE John Elliott, the Government's public affairs adviser, is an extremely busy man it was nothing unusual for him to be the last aboard when a group of VIPs were taken to inspect the on-off Chek Lap Kok airport. But there was certainly something unusual about his impulsive decision to turn around and get off even before ''Captain'' Philip Bruce of the Provisional Airport Authority could set sail. His decision to step back ashore was apparently because it was slightly overcast at that time and he didn't think he would get a clear view of the site. The rest sailed on - and a productive time was had by all under the clearest blue sky Hongkong had seen for months. Let's just hope that as Mr Elliott drove back to his Central office he was not entertained on the car radio by hits such as I Can See Clearly Now, On A Clear Day or even Play Misty For Me.