Hello Matt
Thank you for answering so quickly.
I've just switched to my laptops sound card and it shuts down without any ill effects...
Switched back to SSL2+... and the problem has disappeared!
Marvellous! Thank you so much.

"I see you’re an old timer on the forum."

Definitely an old timer! Although not a regular on the forum... Always amazed at how helpful you all are.
Thanks again for suggesting the cure.