Updated: I have been going through the "Fat Kevin" example and trying to find what I've done differently, as that example has a similar setup (i.e. the life icons in the top left corner).
I noticed that that project had an aspect ratio set up, where as mine didn't. After setting the aspect ratio, the actors now stay in the same relative place as the viewport resizes, so i believe this has resolved my issue :)
Thought I'd leave my solution here in case anyone else comes across this later.