This is certainly your AV app preventing write access to that folder. A new ck2game.exe is not recognised as okay by it. Perhaps @bestbrian can help you with details for Avast.
Thank you Andrew, I tried turning off my AV entirely last thing yesterday rather than just trying to create exceptions and that for reasons I'm not entirely clear on, worked. I was able to launch the game through once, turn the AV back on whilst ingame, shut the game and then restart it with AV on and no problems.
Thank you again for your help and I hope if anybody is having this problem that my solution fixes it for you too.