Went through the official "set up a custom 404 page" which was informative, but then found this as the second listed page down and wondered why I'd wasted time. Works instantly, installed and up and running in less than 5 minutes. Thanks!
Just transferred my site from my localhost to the clients server, I read about all the manual ways of doing it which were a bit tedious. The ease of this extension made it a really quick process - using the backup and the kickstart download from the same page. Really happy - cheers