Thanks for this Patrick,
I will try re-installing and see if the issue is resolved.
Kind Regards
Update:
I followed a step in this ticket
https://ww.wp.xz.cn/support/topic/fatal-error-causing-wordpress-wp-admin-login-crashed/
although my folder was named “Hummingbird Performance” instead.
and have managed to deactivate the plugin, but am very weary about re-installing it! What if this happens again, it seems to be a common issue which is upsetting as I was really enjoying the plugin.