Some progress today it is not the speaker, as the distortion that had been occurring at volumes above twenty cleared so there was no distortion on all sources right up to the max. Not entirely sure what the problem was and remains to see whether it is intermittent. Trial and error to see what causes it but reset the MMi media driver and turned off all the Bluetooth devices. A colleague at work has also had the same issue.
Turns out it is a software problem resolved by doing the reboot on the radio , menu + big dial + top right soft button. Will get Audi to look at on the first service.