Open /etc/apt/sources.list.d/linuxlite.list as Administrator and replace each instance of xenial with citrine.
Do the same with /etc/apt/sources.list.d/linuxlite.list.save
Thanks yet again Jerry, updates now working ok.
As I have shared the iso with U3A members, can you help me to understand why this issue occurred with a fresh install? Did I download an outdated iso (I think it came from sourceforge). I might be subject to lots of 'tuts' at the next U3A meeting after convincing everyone that 'linux just works'
I must be sure of having the latest iso in future.