I tried setting update channel to 8.0 but no updates were showing, will have another go today.
I used a Window 10 machine to flash the image to the CM3 eMMC.
I followed the instructions here: Flashing the Compute Module eMMC - Raspberry Pi Documentation
Had to reboot Windows 10 to disable driver signing.
Then used Win32DiskImager to write the .img file to the eMMC drive.
It was a bit of a faff to get it done but got there in the end.