Thanks for the replies.
I've tried Sidesync. It does much more than I need and less than I want - a simple pop-up to say my phone is charged.
Thanks for the suggestions.
I used to own a Sony Z3 Compact phone.
When I charged it from a mains outlet, I had a notification pop up on my PC when it was fully charged (the phone was not connected to the PC).
Can anyone tell me how to get the same message for my S7?