Since my last post, an interesting development.
When I was trying to get a screen shot, the only way that I could do so was to create a new email so instead of the longer one which I was originally trying to send I created a brief one. When I tried to submit this it actually worked OK and a message that I didn't intend sending was sent and received!.
Having got that far, and thinking that the problem had resolved itself, I tried sending the full original message. This time it failed again.
Leads me to wonder if the problem is caused either by the length of the message or a time limitation.
Gordon W