Thanks Captain
I would like to think something that simple would have been caught by the Support person of my ISP when I called them earlier today. So I called them just to double-check and as it turns out sending an email to a dozen or so people through this ISP is no problem at all. But while I was on the phone he asked me to send the email again and he would try and catch it to see if he could see anything amiss. And he did. Apparently an incorrect email address killed the whole send. I removed the email address and the email went through fine.
Shame Windows Live couldn't have told me! But apparently these error ID's are quite non-specific. To complicate matters the original sender of the email did not experience any problem (obviously, because the original email reached me) so I thought al the email addresses would be ok. The original email came from an academic source and apparently they're not a s fussy as my ISP!
Anyway, I got there in the end so thanks very much for your help Captain. Gratefully appreciated.
I have learned something today: that if one Support person can't help...call back later and try another.