OK. Solved !
A simple reboot of my modem/router (the fibre one from BT - BT Infinity) fixed this for
both my iPhone and Macbook Air.
I can also mirror my iPhone screen to the AppleTV now too.
(mirroring only worked from my Macbook Air before)
That must be a new feature. Excellent !
I guess I should have tried that before, but maybe this will help someone