Still no luck with my fiddling, so I’m posting some screenshots of my setup in the hope someone will be able to find what I have wrong.
The metadata field I am using is a Select field (not thesaurus) for this. (Single select)
I thought maybe I was overthinking it with using the variables on the nested content container - maybe all I need to use is the %fronend_asset_metadata_Type% keyword, since I am wanting to find those tagged the same as the page being viewed. But I have also tested by trying to pass a GET variable to the Nested Search Page.
The Nested Search Page is nested into a Paint Layout, which is applied to the News items. Since I was attempting with the %frontend_% keywords, this should have avoided PL problems that sometimes occured using %globals_%
Search Page Fields setting:
Search Page stored search:
Search Page is set to show all asset status’, and to show News items.
I have tried setting the assets live, just in case, but did not help.
I have tried changing the keyword to _value% and _key% just incase it was trying to match the opposite halves, but not luck. (I also double-checked by making Key & Value the same).
I have set my root node, as per normal. No other restrictions have been selected.
I am hoping this is something obvious that jumps out at someone. 
Thank you again for helping!
Emily