404 Page not working – just shows code

Hi
I have recently moved a WordPress site from a test domain to the proper one.

However, my 404 page is no longer working. Instead I just get a page of HTML which begins at a seemingly random point in my main menu.

Following research into how others have solved the problem, I have:

1) Deactivated and reactivated all plugins

2) Checked .htaccess. This is what it says:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress

3) Changed my permalinks from /%postname%/ to default and back again

4) Validated my site with W3C

5) Reinstalled WordPress

6) Cleared my cache after each step

None of these things has had an effect.

Can anyone help?

Thanks