Ah, right, I finally get it. This is in fact the default setting for Macs on mains power, I think. In Energy Saver: computer never sleeps, display sleeps after 20mins. (And display sleep is not the same thing as screen saver.)
I won't need to change any preferences after all, and the only habit I'll need to change is closing the lid of the MacBook overnight.
Thanks for your patience
