IIS7 custom 404 not showing

created a new IIS7 web site with Intergrated .net 4.0 app pool.

URLs ending with .aspx do show custom 404 anything else gives the blue server error page "HTTP Error 404.0 - Not Found The resource you are looking for has been removed, had its name changed, or is temporarily unavailable." (so nothing to do with IE)

<customErrors redirectMode="ResponseRewrite" mode="On" defaultRedirect="/pages/404.aspx" />
</system.web>
<system.webServer>
    <httpErrors  >
        <remove statusCode="404" subStatusCode="-1" />
        <error statusCode="404" prefixLanguageFilePath="" path="/pages/404.aspx" responseMode="ExecuteURL" />
    </httpErrors>
</system.webServer>

also tried

<httpErrors existingResponse="PassThrough" />

but that just resulted in an empty response.

I have only found one reference to the usefulness of executing the appcmd to test the custom http error handling but here are the results.

C:\Windows\System32\inetsrv>appcmd list config "http://mysite/file.notexist" -section:httpErrors

<system.webServer>
    <httpErrors>
        <error statusCode="401" prefixLanguageFilePath="%SystemDrive%\inetpub\custer
r" path="401.htm" />
        <error statusCode="403" prefixLanguageFilePath="%SystemDrive%\inetpub\custer
r" path="403.htm" />
        <error statusCode="404" prefixLanguageFilePath="%SystemDrive%\inetpub\custer
r" path="404.htm" />
        <error statusCode="405" prefixLanguageFilePath="%SystemDrive%\inetpub\custer
r" path="405.htm" />
        <error statusCode="406" prefixLanguageFilePath="%SystemDrive%\inetpub\custer
r" path="406.htm" />
        <error statusCode="412" prefixLanguageFilePath="%SystemDrive%\inetpub\custer
r" path="412.htm" />
        <error statusCode="500" prefixLanguageFilePath="%SystemDrive%\inetpub\custer
r" path="500.htm" />
        <error statusCode="501" prefixLanguageFilePath="%SystemDrive%\inetpub\custer
r" path="501.htm" />
        <error statusCode="502" prefixLanguageFilePath="%SystemDrive%\inetpub\custer
r" path="502.htm" />
    </httpErrors>
</system.webServer>

which is odd because in iis7 manager the error pages show

404 /pages/404.aspx Execute URL Local

.Net Error Pages shows nothing, though I did have an entry in there.

Question 1 : what steps do I need to take for a completely new asp .net 4 iis7 site to have a custom .net error page for every 404 result ?

Question 2 : why does the .net handler work for .aspx files and nothing else ?

note: set the 404 at server level and the appcmd command then showed the custom 404 in the path, but made no difference to the site failing to showing 404.

So I am guessing it's a red herring.