Thanks for your reply. The site is only HTML, so it's interesting that you say web.config is associated with ASP.NET. Maybe that's a clue.
The first line of the reply form looks like this:
<form action="http://www.artmongers.com/cgi-bin/formmail.pl" method="post" onsubmit="return validateform();" id="contactQuery">
The thing is, it always worked fine on the old server and nothing was changed.
I have another site using CDO and VBScript to send an email, and that's stopped as well, complaining that the 'SendUsing' configuration was wrong. I did find something on MSDN that said there were changes when moving to IIS7 and tried implementing their code, but got a syntax error I can't work out when trying to set the SMTP server. All very confusing, but it seems likely that both problems are to do with the change of server, IIS version, or both.
If you're any the wiser, I'd love to hear from you!