I'm sure I've missed something silly but can't find what it is.
Have a domain called jslondon.org (which works fine) - decided to add support for www.jslondon.org by adding a cname and updating the apache definition with www.jslondon.org set as an alias.

www.jslondon.org does not resolve.

Worth noting that I've done the same with a different domain (gnesher.com) which works perfectly - so I'm obviously just missing something :/