Wouldn't you know it? Just after posting here about my proposed charge regime for the M1 MacBook Air it stopped charging. By a process of elimination, I deduced the supplied USB-C cable was crook & only connecting intermittently. Temporarily rectified by using a different cable, I have no shortage of those. I'm actually using the Sugru-repaired cable from the 2017 MacBook Pro.
I contacted Apple Support via the iPad app and eventually an appointment was made at the nearest Apple Store (18km away) for Wednesday afternoon, which was the first available time-slot. Under Covid rules, Apple stores are only open by appointment.