26th December 1971.

When RNI's International Service started broadcasting a British Top 20 in August 1971 (see here), the chart was similar to, but not exactly the same as, the official chart published in Record Retailer magazine (soon to become known as Music Week).
 
By the time of this chart, three months later, it differed significantly from the official chart and included a number of records which weren't genuine British hits at all, like those listed below by Frankie Howerd, Curtis Mayfield, Gene Pitney and Redbone.
 
This chart was obtained from a recording shared on Azanorak, confirmed with information from radiopedia.nl, with grateful thanks to both.

Robb Eden, from the second hour of the British Top 20 show on Radio Northsea International, 26th December 1971. This clip is taken from a recording shared on www.azanorak.com. Our thanks to Ray Robinson (duration 2 minutes 36 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 British Top 20 from 22nd August.
The next is the Prediction Hit 40 from 4th March 1972.