Once montior has blanked (e.g. power saving or lock session) - cannot be restored

Using Xubuntu 18.04.3 on Q190 Lenovo desktop connected via HDMI to monitor Benq GW2760S.

if power saving blanks the monitor or I "lock screen", the screen will blank - but neither mouse movement nor keyboard pressing will bring the display back. Monitor displays "no connection". In this state, my only option is to hard boot the machine - which is not good.

As a work around, I've had to:
- set the power button to "shut down" via the power config, so now at least I can shut down in a clean manner if screen blanks
- turn off power saving
- do not lock my screen

However, being unable to lock my machine leaves it vulnerable!

My monitor's power saving system has been turned off, DDC is on and HDMI Auto Switch is on. I was using Mint Xubuntu and more recently Lubuntu 19.04 - I believe both of which had no issues with the the monitor.

I would like to stay with Xubuntu, so any help would really be appreciated.
Marc