OK, it seems 
	Code:
	Source template:{tags}
Destination field:authors
 does the trick.
How can I truncate the author_sort here
	Code:
	Source template:{author_sort}: {title}
Destination field:title
 to the first author's surname, please?
I found this somewhere
	Code:
	{author_sort:re(([^\,]+)(\,|$)(.*),\1)}
 would it do the trick, and do I really need all of these brackets. I can't find explanations for them in the 
online help.