Thanks PH ...
Indeed I should have stated previously that:
correctly gets replies. So it does resolve outside Apache2. But maybe Apache2's error message is misleading.
Regarding nslookup: I am running Debian Wheezy 7.0.
So far as Debian is concerned, nslookup is part of BIND and all the indications are that it cannot be installed alone. I don't wish to install BIND as I already have a DNS server on my Windows 2003 R2 domain controller.
The machine name, and all virtual host names are entered into the /etc/hosts file (because ultimately I am intending to run this machine as a standalone web server) so far as direct access on the local machine is concerned.
I have a couple of other things I will try, but what I need to avoid is a repeat of what has already happened - namely that the server became quite "messed up" and reinstalling packages didn't sort it out, so I had to reinstall the lot: OS, Proftp, Apache2, Webalizer, MySQL, PHP5 etc etc etc!
Thanks again for your attention.