Unfortunately the few times I'd briefly tried to do it I'd hit a dead-end as the answer wasn't obvious; the online help was outdated and clicking on any configuration options within each domain failed to give any clue as to how to do it.
The need became more pressing so I investigated further and it turns out that you cannot add a subdomain to a domain, you have to add a subdomain to your account. In essence, it is just like adding a new website to your hosting system.
How to add a subdomain to your domain.
- Log into FastHosts Control Panel
- Click on "Add Domain"
- Click on "Add Subdomain" in the list
- Enter the subdomain title and click on the domain picker
- Select your domain
- Click continue to configure the subdomain.
For more detailed information, see this link: http://www.fasthosts.co.uk/knowledge-base/?article_id=858