Thanks for taking the time to respond. Yes I did check select colour option and I'm using the Mac 2019 version (as specified in my post). Unfortunately I can assure you it is not working on the version I have which as far as I can tell is the latest one that I downloaded yesterday.
It could be an issue unique to the Mac version. Sorry I can't be of much help there. If you look in the Keying effects, is there an Ultra Key effect? I'm not sure if it's on the Mac version, but it should produce an excellent key shot.