OK,
So for anyone who is interested, hello? hellllooooooooo (echo)
I've managed to do everything I needed without any Ajax in the end. I used GreyBox (didn't like the default css themes on colorbox and the GB with SS looked fine) and jQuery.

1. Render a form into a javascript popup with a thankyou message on submission.
An HTML <li> element on the main page has a javascript function 'launchSignup()' as it's 'onclick' property.
The javascript function calls GreyBox with the location of a virtual subpage of the main page
The virtual subpage function just re-renders the object with a new template file containing the form variable and different content
the form action function does its thing and re-renders the object with a thankyou template displaying more content from the CMS

2. Render HTML returned from an external service into a javascript popup
An accountID input field on the main page hold the account ID
An action button just below it has a javascript function call as its onclick property
the javascript function calls GreyBox with the URL mainpage/balance/accountID
the balance function calls the external service with the accountID and receives an HTML response which is stored in a local variable
the balance funcrion returns renderWith('BalancePage.ss') which has a reference to the local variable to render the result.

Easy once you know how :o)
All done from a single slab of PHP and a light sprinkling of javascript. Isn't SS wonderful?