You can only have one domain per package, but of course you can have sub domains aplenty.
so each sub-domain would be http: // subdomain. yourmaindomain.co.uk
I've added spaces in the address to prevent the site turning it into a 'click here' link
Each subdomain must have its own document root inside the public_html directory, and browsers will load the index page that corresponds to the subdomain. So, all files relating to a subdomain called pcadvisor must go into a directory called pcadvisor inside the public-html directory.
You should never use your virtual server space to host websites for other people by the way - that's a practice that's fraught with danger.
and use it to host many (but not all) of my clients' websites. It's ideal, because there are virtually no limits to the possibilities, but it's not cheap, and unless you are going to host multiple domain names it's probably not a viable proposition.
I must say that I've been delighted with Heart internet, they are without doubt the most professional hosting company I've ever worked with.