Have you checked the DNS status for you domain with dnstools.com or some such? I'd look there to determine for sure that DNS has updated.
Secondly, if you somehow got the files uploaded to an incorrect directory you could get a "not found", though if the domain is working correctly and there are no files in your web space, when you browse directly to your domain's "base" directory (http://yourdomain.co.uk/) you should be seeing an empty directory listing.
Finally, if you have files uploaded into your "base" dir, and the index.php that loads by default immediately calls another file which cannot be found (because of phpb3 configuration issue?) you may be getting the not found message.
Without knowing your domain name so we can check a few things ourselves, it's really hard to guess what might be going on.