How to stop the multisite root address appearing on sites

I have a multisite called http://www.bothybiz.com. One of the sub domains is 18u, so when someone looks at the 18u website, they are looking at http://www.18u.bothybiz.com. 18u has its own domain name http://www.18u.org.uk which has a redirect to http://www.18u.bothybiz.com. Is it possible to stop all of the pages on the site having bothybiz in their page addresses?

All of my sites are hosted through Heart Internet.

Any advice would be appreciated.

Thanks

Aileen