JohnW,
"I'm new to this working on a Windows10 system : have downloaded the beta file which is a .dmg file - will not open or launch. Guidance please."
There are actually two installers. One is the .dmg file which is for Apple products and then there is a .exe file which is for Windows. Ensure that Windows 10 1611 Patch or later is installed for the software to be able to function correctly in Windows environments. I tested on varied deployments and anything before the 1611 patch is really buggy and can have errors upon installation.
Please visit:
https://affinity.serif.com/en-gb/windows/and download the correct installer for Windows.