Dunno but Google 'sed only first match' gives some answers...
I resorted to just using a perl one liner. It wasn't as straight forward as that. Using sed's {p;q;} syntax with the pattern provides the functionality, but it was the capture group only I was interested, and not the whole line which I couldn't make work with those commands.
Perl was simpler, don't know why I didn't think of that initially.
Thanks, jlc