.....and normally, when a site is added to a server then it is added as just 'yourdomain.com' and the www. is automatically added as an A Record. It seems that your server works in reverse and that www.yourdomain.com is set up but that there's no record for just 'yourdomain.com' - which is why it reverts to a server SSL Certificate if you try to use that address. That's something you'll probably have to get Heart Internet to sort out.

Vger