Okay, I'm so very nearly there.
I've published the library to Dropbox in the updog directory and created the _catalog etc.
I can view the library at
https://exampleme.updog.co with a web browser & passcode but I can't seem to make the connection via my iphone & Marvin.
I've entered the address (and my passcode that I created in updog to access the "site"). I've also tried without the passcode but no joy.
When accessing via Marvin SxS, I get a blank screen.
thanks again!