So the first step to resolving this for me was to read over this post on the official google groups support forums which pretty much consists of a bunch of users complaining they can only see as I do, Art Gallery and Experimental but no Google Photos options!
After a little further digging I came across a video by Tech with Brett
While the video breaks down how to enable the feature, it doesn't really give you any insight into how to bring it back if it's just happened to vanish - which is the case for myself, it was present on day gone the next.
In the video he shows how to set the TV to make use of BACKDROP - this needs to be set not he tV side of things, see this article on Google Support which details how to enable backdrop - the specific part of interest for you will be "Fix issues with your screen saver"
After making sure our TV is using Backdrop as screen saver, start the screen saver on the TV and while it's in screensaver mode showing you the usual Google images, now open the Google home app and navigate to your Television of choice, in the top right click the gear icon then into Ambient mode, you should now see Google Photos as an option.
TLDR; The key was to actually have the screensaver running to see the Google Photos option, from here while it was running I was able to select an album, see the changes applied automagically and I can still see the option for Google Photos even after exiting the app or turning the TV on/off.