Well, this morning I was looking for something else but decided to check out my settings again for sound for incoming Mail. I did two things and since then - I'm getting a Basso sound for incoming Mail.
The first thing was in System Preferences and went to NOTIFICATIONS where I changed from ALERTS to BANNERS and made sure that the 'Play sound for notifications' box was checked.
The second thing I did was go into SOUND and unchecked 'use ambient noise reduction'.
I'm not sure whether either of these has made a difference but it is coincidental that I am now getting a sound........three arrived whilst I've been typing this....fingers crossed that it stays with me.