SQL 2012 SP2 There are no SQL Server instances or shared features that can be updated on this computer

Hi, 

I have a SQL Server instance:

Microsoft SQL Server 2012 - 11.0.5058.0 (X64) 
May 14 2014 18:34:29 
Copyright (c) Microsoft Corporation
Standard Edition (64-bit) on Windows NT 6.1 <X64> (Build 7601: Service Pack 1) (Hypervisor)

I am trying to install:

SQLServer2012SP2-KB2958429-x64-ENU

On the "Select Features" screen of the setup I get an error message when I try to click next:

"There are no SQL Server Instances or shared features that can be updated on this computer."

I've checked and re-checked the release notes - pretty sure I have the right versions of everything:

https://msdn.microsoft.com/en-us/library/67cb8b3e-3d82-47f4-840d-0f12a3bff565?f=255&MSPPError=-2147217396

Any help appreciated!

Cheers

Lucas

LucasF