Screen Mirroring problem

It’s a two-month old iMac 2017, High Sierra.

I am trying to screen mirror to a Samsung TV with Airplay built in. (Also two months old).

There is no screen mirroring icon in the menu bar. I checked the box under preferences, displays. 

When I click Bluetooth in the menu bar, the TV appears, so the iMac ‘knows’ it’s there.

Two iPhones and an iPad all screen mirror instantly with the tv, so the tv is not the problem. 

Everything is on the same wifi network, all software is up-to-date.

Everything, inc modem has been rebooted.

There is no firewall or other security that I’m aware of.

Still, no mirroring icon in the menu bar.

I bought this setup largely for screen mirroring, and now it doesn’t work. 

Apple is Hard to reach with Covid19 and now Easter.

I assume there’s probably a box somewhere deep down in settings that i have or haven’t checked, but I just don’t know. Any suggestions out there?

Many thanks 

Tom