Ray, thanks for looking at this, From the form submission of the cms index.php page, a successful login should take you to cms.domain.com/pages/ folder, your amendment makes it clear that the page is instead taking you to cms.domain.com/index.php, which is a .htaccess mirror of the domain.com/index.php page. I think something is missing from the .htaccess allowing other pages in the subdomain other that index.php, /pages/index.php for example.
I've now updates the .htaccess record, which stops the redirect, the form on the CMS login page now redirects successfully.
Many thanks again for looking at this.