The point was that the 404 error wasn't being handled gracefully, but I see it is fixed now.

Note: Links to LDWA website pages are case sensitive, so the correct link is http://www.ldwa.org.uk/strider/, as we're using a Unix based server to host the LDWA website.

Type in something like http://www.ldwa.org.uk/STRIDER
which is clearly wrong, but who left the caps lock on?
You get

Not Found
The requested URL /STRIDER was not found on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

i.e. the page wasn't found (correct answer), but it couldn't find the file specified in the ErrorDocument in the .htaccess (is my bet) to handle the "Not found"

Oops!