Radio 2 to Radio 1 FM Handover - January 1989
As recently as the mid 80s, Radio 1 didn't have it's own FM Stereo network, and was only available on Medium Wave for much of the time.
However, they were allowed occasional use of the Radio 2 FM network, and this was generally between 10 and midnight on weekdays, 1.00pm - 7.30pm on Saturdays and 5pm to midnight on Sundays.
Radio 1 launched it's own FM network, starting in London in October 1987, then had a big switch on for 60% of the country on September 1st 1988, with Bros switching on transmitters across the country.
For those areas without Radio 1 FM, they still borrowed the Radio 2 FM transmitters for the Top 40, and that is where this example of the handover comes from, from January 1989