It's not a file system permissions issue or an incorrect secure/unsecure base URL. It's probably an incorrect Base Skin URL. Here's an example of your stylesheet link:
<link rel="stylesheet" type="text/css" href="/home/sites/isaacgrainger.co.uk/public_html/skin/frontend/base/default/css/normalize.css" media="all" />
The problem is the href attribute. /home/sites/isaacgrainger.co.uk/public_html/ looks more like a path in your file system than a web address.
There are a lot of ways this could have happened, but the most likely and least painless would be if your base_skin_url was incorrect.
If it is, you can fix it with this SQL query:
UPDATE core_config_data SET value='{{secure_base_url}}skin/' WHERE path='web/secure/base_skin_url';
UPDATE core_config_data SET value='{{unsecure_base_url}}skin/' WHERE path='web/unsecure/base_skin_url';
Or if you have admin access in the backend, you can change it there too:
System > Configuration > General > Web > {{Unsecure||Secure}}
Find the fields labeled "Base Skin Url" and change them to this:
{{secure_base_url}}skin/
{{unsecure_base_url}}skin/
(secure for secure, unsecure for unsecure)