It could be one of the sites on the Sage UK domain.
Many businesses run mini-websites with addresses along the lines of companyname.sage. co.uk
You can find out more about this here:
At the end of the day your client has to furnish you with the necessary publishing credentials, unless of course you arrange hosting for them in the first instance. Unless they provide these details you obviously cannot upload the finished site, but I can understand how someone might want to see things up and running before paying for them.