Tried again. This time they suggested an SNC reset and it seems to have done something so I'll have to wait and see if the solution holds up this time.
I do hate intermittent faults.
SMC reset went like this
1 shut down Mac
2 on the built-in keyboard press and hold your Mac may turn on - mine did
Control on the left side of the keyboard
Option on the left side of the keyboard
Shift on the right side of the keyboard
Keep holding all 3 keys for 7 seconds then press and hold the power button as well if your Mac is on it will turn off as you hold the keys
Keep holding all four keys for 7 seconds then release them.
Wait a few seconds then press power on and log on.
Like I said it's working for now even when I put the Mac to sleep and logged off music and back on again. I don't have any downloads so I can't say anything about that.
This applies to M1 MacBooks only
The support article is for Intel Macs