21st October 1972.

Some of the details of this chart were obtained from a recording shared on the Offshore Radio Club Forum by Harm Koenders, supplemented and confirmed with information from radiopedia.nl with grateful thanks to both.
 
This chart was only used for a short time because Radio Northsea's International Service came to an abrupt end on 24th October 1972 when, withour prior warning, it was replaced by Dutch programmes. However the English shows returned on 3rd November. More details here.

About twenty minutes into this show Dave Rogers announced that he had just been informed that the station was having to go off the air at 8pm for “essential work on the antenna” so he only had a one hour programme. For the last forty minutes, he just played the tracks going up the chart. Recording courtesy of the Offshore Radio Club Forum:

Dave Rogers from the Prediction Hit 40 chart show of 21st October 1972, recorded from the station's FM outlet (duration 6 minutes 21 seconds)

The links from the songs on this page lead to the Amazon MP3 Store where you can hear short audio clips of the tracks and customers in the UK can buy downloads. Please see this note.
 
There are a number of other offshore charts listed on The Pirate Radio Hall of Fame. Full details here.
 
The most recent previous RNI chart we have is from 15th July 1972. The next one is the Top 30 from 22nd December 1972.