It appears that it has fixed itself and installed i think. I just double clicked the setup.exe and it appears to be now installed instead of giving me an error so presumably if i had restarted my computer it may have worked although i did check multiple times on Task Manager to make sure nothing to do with itch.io was running before attempting to run the setup.exe again. Maybe it was some hidden process running to do with itch already after i had pressed to install it the first time.
Thanks for your help anyway its much appreciated :P