Why I cannot open next.php-fusion.co.uk

Yes; it's a hosting issue. Try http://next.php-fusion.co.uk/ (without www.) instead.

What solved this issue for many of us (thanks to kneekoo and a meeting):

Open cmd.exe as administrator.

Type in the following line (or copy and right click paste):
attrib -r C:\WINDOWS\system32\drivers\etc\hosts
It makes the file writeable.

Open the file above (in notepad), and add these lines:
178.18.82.175 next.php-fusion.co.uk
178.18.82.175 www.php-fusion.co.uk

And then type this line to the cmd:
attrib +r C:\WINDOWS\system32\drivers\etc\hosts

It's not the best solution, but noone knows what's the problem, therefore it can't be solved.

quite often the difference between being able to use www. and not, has to do with the way the domain is registered by the registrar. It is something for the owner of that domain to take up with the registrar and get the MX records updated. But in the meantime, making a modification to the hosts file is a great workaround. :)

A workaround that should not have to be necessary, but appreciated nevertheless.

Thanks for the replies. I wanted to ask this question because sometimes it opens with/without "www" sometimes it won't and sometimes neither of these two options is possible to open the site. It's a pitty for me not to be able to open the development support the easy way.