Great, it's working.
I didn't have/any experience or DDE which turns out to be what I needed to know. Chris Davies gives the answer in his post (26) here:
http://lavag.org/topic/4851-associating-my-exe-with-a-specific-file-extension-type/page__st__20
To open a file once the app is open, the Dynamic Data Exchange is used which requires a few more keys to be added to the registry (he lists it). The end result is that the OS Open Document is called and everything works swimmingly
Thanks,
Martin