and the tag is what tells the domain name servers where your domain name is located.
The DNS are located at different points around the world, and they maintain vast databases which record the location of every registered domain name. When you type the address of a web site into your browser it's the Domain name servers that point it to the server where your name is hosted. Each web hosting company has primary and secondary name servers, and these hold databases of all domain names hosted by the company. Each company has its own 'tag' and this is normally the name of the company itself.
Only the registrant (in this case you) of a .co.uk name may request a retag, and what you must do is request the company with whom you registered the name to retag it to HEARTINTERNET. In most cases the receiving company will contact the sending company and request the retag for you, all you have to do is tell the sending company to make the retag.