Sorted it. The progress bar is with the .deb package, but not before a bit of backwards and forwards.
First install ClamAV using these commands from the project's GitHub page:
sudo apt install python3-gi python3-gi-cairo gir1.2-gtk-4.0 gir1.2-adw-1
then
sudo apt install python3-dev libcairo2-dev libgirepository-2.0-dev pkg-config
then
sudo apt install libjpeg-dev zlib1g-dev
then
sudo apt install clamav
Now to install the ClamUI .deb package go to this page:
and scroll down the page and download
clamui_0.1.4_all.deb
Once downloaded, either use the default Software installer of Zorin or use Gdebi.
I will include this in the Mk.II of the Unofficial Manual for Zorin 18 Core based on r3.