O.K sounds as if I should just use forcewww. Can I just confirm (sorry I am very out of my depth) - they are asking me to set up the www as an A record to my server IP, and the naked domain as an A record to the IP address they give? So no CNAMES?
What I have now:
A record pointing naked domain to server IP
CNAME record pointing www to naked domain
Proposed change using ForceWWW
A record pointing naked domain to forcewww IP
A record pointing www to my server IP