Mand,
Presumably the memorial you refer to is in Liverpool? When was it dedicated?
Ernest Charles Benn does seem to disappear without trace, but looks worth pursuing. I can't see a marriage or death for him on FreeBMD. If you have access to the full BMDs on Ancestry etc, try to locate a death for him in later years, if only to rule him out.
Are there any AVLs for your area?
It is also worth checking the Scottish BMDs as there were several military hospitals across the border. He may have died post-discharge, without returning home, which means he could be missing from CWGC. One of my Non-Comms probably never left hospital, but died 6 months after being discharged from the Army.
To help you narrow it down a bit, I checked the airmen's index in AIR 78 and also the CEF and AIF records, just in case. I couldn't see anything.
Phil
Edit: Ignore the first bit. I've found it.
http://thewarmemorial.blogspot.co.uk/