An update on this.
I took the head unit out yesterday after deleting my paired phone and forgetting the HU on my phone. Disconnected all the plugs and wires from the HU and left it disconnected while I started the car up. Waited a while and then reconnected everything. HU remembered stations but had to reset the time.
Re paired my phone and everything works again as expected.
I’m sure I’d already tried disconnecting everything previously when the mic issue first appeared but hey ho it works now so not complaining!