Unable to access blog after domain change

The most likely explanation is that your file permissions are set incorrectly. There’s an explanatory Codex article about just that:

http://codex.wordpress.org/Changing_File_Permissions

So when someone visits http://sallys.me.uk what do you want to happen? At the moment it looks like an unconfigured WordPress 3.9.1 site.

You can have one or many domain names pointing at the same place. Set the nameservers for the domains to point to your hosting server, and set up the hosting server to have the additional domain recorded as an add-on domain. If that’s opaque, your host support team should be able to do this for you.

If you’ve got two file areas for the two domains and don’t want to change that, then sallys.me.uk need contain only a single file called index.php which contains the following text:

<?php
  header('Location: http://sallysrollerdisco.uk.com/');
?>

I note that sallysrollerdisco.uk.com is not taken yet, so the above will not work until you have purchased it.

That makes a big difference. WordPress.com have their own way of doing things so you will need to contact wordpress.com support.