By the way Boxbilling doesn't work. It returns PHP errors. I might actually invest in WHMCS at some point as I know for a fact that'll work. Thanks for all the help guys.
I tried The Hosting Tool. It's good for a free software. However it will not suit my needs I'm afraid. So yeah, WHMCS is probably the best bet as that's a great piece of kit. But it's also one of the most expensive ones.
Nevertheless, if you think about it, it's worth it.