Hope someone can spot how I did this, as I'm having a dumb moment and can't seem to remember.
Its not actually monochrome, but you'll see what I mean from the link.
Basically i converted colour images to more muted versions, but can't quite recall how I did it.
If anyone can have a look and immediately know how to recreate it, that would be hugely appreciated!
You can see what I mean hear - original colour image first, then the version I turned them into:
Ignore this - eventually came back to me.
Filter > convert to alpha.
Filter > invert.