Custom File Not Found 404 not being reached.

Hi all

I have created a web page to handle 404 File not found on my MCMS Website.
It is set to be called from the IIS and also from web.config.

It works perfectly fine when I use the server or local host name, e.g.:
http://localhost/richardsite/nonexistant.aspx
http://localhost/richardsite/nonexistant
(nonexistant is really non-existant!).

Problem is when I make an error through the header hostname 
"www.richardsite.org" it fails.  This hostname is set in the IIS - Default

Web Site > Properties > Web Site > Advanced.  It is also named this in the

CMS!

e.g. A) www.richardsite.org/nonexist.asp ------ shows up with a default
IIS 
or CMS 404.
but when I use:
B) www.richardsite.org/richardsite/nonexist.asp ------ it shows up
correctly 
with custom 404.

When (A) is used, it seems to be calling a default IIS or CMS 404, could
it 
be that CMS is intervening and so the IIS is not picking it up ? Is there 
anyway around this or am I simply setting the error handling in the wrong 
area?

Many many thanks for assisting.
Richard Leung