Exch2013 CU6 installation - Another version of this product is already installed

Hi,

I am currently trying to install CU6 to our Exch2013 server but all I receive on running the exchangeserver.msi is:

Another version of this product is already installed.Installation of this version cannot continue. To configure or remove the existing version of that product use Add/Remove Programs on the Control Panel

Has anyone come across this before?

Get-ExchangeServer | Format-List Name, Edition, AdminDisplayVersion

Name                :
Edition             : Standard
AdminDisplayVersion : Version 15.0 (Build 516.32)

This shows the RTM version being installed and I am trying to get CU6 going.

Thanks
Stefan