Adjust the overscan settings in the boot config.
The config settings are described here
https://www.raspberrypi.org/documentation/configuration/config-txt/README.md
Hi Paul
Thanks for your reply. It lead me to find the correct values to change.
For others to find (myself in the future!):
The config.txt is located on the unmounted partition
mmcblk0p1 which can be mounted using:
sudo mount /dev/mmcblk0p1
You can then find config.txt in /mnt/mmcblk0p1/config.txt
I removed the '#' in front of the line #disable_overscan=1 and it did the trick.
Reboot to see if it works.