22nd August 1971.

The International Service of RNI stopped sharing the Super Hit 50 chart with the Dutch Service at the beginning of August 1971. From the 22nd of that month, a British Top 20 show was introduced into the Sunday evening schedule. This is the first of them.
 
The Top 20 chart show ran weekly until February 1972 when it was replaced by the more adventurous Prediction Hit 40.
 
It is not known how this Top 20 was compiled. It differs very slightly from the official UK chart and the ones published that week in NME and Melody Maker (scroll down).

This chart was obtained from a recording shared on The Offshore Radio Club Forum by Hans Hendriks. Our thanks to him.

Dave Rogers starts the first British Top 20 show on Radio Northsea International, 22nd August 1971 (duration 3 minutes 24 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 previous RNI chart we have is the Super Hit 50 from 1st August.
The next is the Prediction Hit 40 from 4th March 1972.