I have seen a few posts about this issue but none of them have useful information about this issue which seems to happen in Fasthosts servers.
The real issue seems to be a server configuration problem, I have no real experience with Fasthosts but it seems that the way their servers are configured clashes with the way Wordpress handles pretty URLs and 404 error page gets cut off by a random number of characters and it's displayed as plain text (HTML source code).
An odd solution that I found was to add a HTML comment tag that wraps a random string, I used 7935 characters (including tag), for the issue to be solved, the number of characters my be different on other Wordpress sites and on different themes.
I'm posting this in case someone else get frustrated by this error and I'd like this to be helpful for others even if it's not really the best solution.
Cheers,
Raul.