1. Download and run the 580 update first. This allows the patch to be utilised by the game engine.
2. Run the game and confirm that it says version 580 in the top right corner of the main menu window.
3. Download the May12 patch. Extract the patch to a folder of your choice. I created a special folder for this patch to make sure that it doesn't overwrite any earlier downloads and that I can find it easily.
4. To run the game updater you can simply click on the game version at top right of the main menu - the line of text up there.
5. Once the game installer runs, I uninstalled the earlier update, DLCs, etc. This is not necessary - just being careful.
6. Next, click Install at bottom left of the game installer window.
7. Browse to the folder containing the downloaded patch.
8. Open the CORE folder and locate the desc.addpack file. Select it.
9. Wait until the install completes.
10. Repeat the process to download and install the operations updates (optional). This file is over 1GB. I installed this.
11. Click Save & Play or whatever it says - my game is running at the moment.
Probably the toughest patching process I have ever seen
but I haven't had any crashes yet - fingers crossed...