Ok, I think I've got it now, thanks Lynda.
From this page http://kb.worldsecuresystems.com/kb/secure-urls.html#main_Linking_to_pages_that_contain_fo rms_with_secure_URLs
To create links to secure pages, follow these steps:
- Create the form and insert it on a web page (such as a page with the URL, www.my_site_name.worldsecuresystems.com/register.html)
- Create a link in your site that references the SSL version of the page, like this: https://my_site_name.worldsecuresystems.com/register.html
Have just tested this and it appears that this is indeed the case, the page with the webform on it that needs to be secure is on my_site.businesscatalyst.com/web_form_page_collecting_sensitive_info.html but all links to it are using my_site.worldsecuresystems.com On clicking through to it the page url is https. So that's how it's done.