How to get Mail files from Mountain Lion Time Machine backup in Mavericks?
Sorted it after a long trawl through the internet. For any one else with a similar problem...

Thanks to this post by Alberto Ravasio here


  Last but not least, navigate the TM disk within the Finder, (do not enter TM).
  When you are on the date you are looking for, navigate the directory structure until you reach the Users folder.
  
  Open Terminal and type

cd

  
  and a space, drag and drop your user name folder inside the Terminal window. That completes the cd command for you. Hit return.
  
  You are now into your user folder inside the TM disk. Type

cd Library
open .

  
  please note the dot after open. Last command opens a Finder window of the Library folder inside TM disk.
  
  Now you are ready to get what you need.