We have NAMED serving internal (only internal) DNS for our domain; let's
say company.com. It works fine for server1.company.com,
pc123456.company.com etc. Everything else is forwarded out to our ISP's
DNS server.

We also have a website, www.company.com, which is hosted externally.
I've put in an A record for www.company.com into the company.com zone,
but if we try and access www.company.com internally, we just get our
ISPs holding page. If I try and access the website by IP address
externally, I get the same thing, so our ISP must be doing something
clever with multiple sites on the same IP.

How can I tell NAMED to pass on the lookup for www.company.com to the
ISP's DNS, rather than trying to resolve it itself?