Hello Everybody,
I will get straight to the point, which the title explains neatly.
When I run the install (I have the Latest GOG.com Downloader and Fully Updated game) I get this message:

(Ahem)

The source file is corrupted

You should run the installer's integrity check to verify which files need
to be re-downloaded

I have re-downloaded many times but it still does not work

Please Help!

-WarpDrive

Turn off the downloader and download it again, if you then still have problems send the issue to gog support.

Might be a problem with your connection if you keep getting corrupted downloads. Tried turning off any virus software and firewall software for the duration of the download itself, just to prevent them from having any possibility of interfering with the file as it downloads to your machine?

Thank you for your help, I will try both your solutions and I hope they work,
Thanks again.