IIS uses <httpRedirect>
with a variety of child elements to achieve the required effect, if necessary.
It's documented here: http://www.iis.net/configreference/system.webserver/httpredirect
In the filesystem directory corresponding to the /something/
path for your domain.co.uk
website, add a web.config file with this:
<configuration>
<system.webServer>
<httpRedirect enabled="true" destination="http://www.newdomain.co.uk" exactDestination="true" httpResponseStatus="Permanent" />
</system.webServer>
</configuration>
Note that your domain.co.uk/something/
and newdomain.co.uk/
directories cannot share this web.config otherwise it will cause an infinite redirection loop. If you want canonical domain names then I recommend using the URL Rewrite module instead, which provides redirection abilities in addition to URL rewriting.