It gets to 96% at the "Executing Setup" part and then fails. I downloaded the manual update and that crashes straight away!

I haven't tried the Offline Installer files yet (I have downloaded them, just not used them yet)
I used GOG Galaxy to grab the update and the game seems to be running fine here.
I have launched the game with GOG Galaxy running and the game was fine, and a second time without GOG Galaxy running by launching the games exe directly (same as if using the Offline Installation) .. and the game ran fine again.

Sorry I cant be any help, but I am just chipping in to say the download may not be at fault here.

I had the usual post update cache graphics refreshing related occasional stutters in the first few minutes, but the game after the update has settled in has been as smooth as ever for the last hours play, and subsequent reloads after the first were all fine too.

Have you tried the verify/ repair function on the game before you install the update?

My last offline updater went ok, but you need beforehand to have the right previous version installed for the update to install, otherwise you should get 'wrong version' or similar message. Problem with offline installers is you must always follow the right sequence of updates, if you skip downloading one you cannot use them anymore, updates will get out of the list at some point. You must then download the main oflline game installer files which is usually 8GB+ filesize.

Thanks for all the help guys! I finally got it installed via Galaxy. What the heck was going on, I don't know! Next thing is waiting to see what happening when the 3.10 drops some point in the future!