I had to give support a shout about having no mouse control at all and they gave me a bit of info to try and it worked fine, decided to add it here for anyone else that may come across this issue in the future.

Issue :

No mouse control whatsoever in the menus or whilst playing, mouse pointer had a black box round it on menu but wouldn't move. Once in game no mouse input at all either in car or on foot.

Fix (copied from the email and credit goes entirely to GOG support) :


I just had to do the 2 Direct X Fixes on stage 3 and that fixed the issue.



Hello

1) Please make sure that you use "Run as Administrator" to install and to start the game (right-click on the shortcut or setup file and select "Run as Administrator").

2) Please reinstall the game and add it to your anti-virus/firewall software's exception/trusted list.

3) Install the June 2010 DirectX runtime package, then use the DirectX web installer.

You can find the archive file containing the 2010 package installation files here:
http://www.microsoft.com/en-us/download/details.aspx?id=8109

You will have to extract the files into some folder, then open that folder and launch the DXSETUP.exe file in order to proceed with the installation.

Afterwards, download the latest DirectX web installer and use it as well:
http://www.microsoft.com/en-us/download/details.aspx?id=35

4) Please make sure that your graphics card drivers are not forcing any settings upon the game. To do this, right-click an empty area on your desktop and go to:
NVIDIA Control Panel -> 3D Settings -> Manage 3D Settings
and click “Restore”.

Regards
Genoan

GOG.com Support