Hello,
After many hours I found out that its impossible to have a Form group within a form group in the Moodle forms API and standard HTML markup does not allow elements within radio inputs. I've gone for a checkbox / radio solution with disabledIf logic. It did find out though that disabledIf does not work well with two radio groups within an addGroup. All being well when the job is done the code will be within a plugin on Moodle.org for all to see.
Cheers,
Gareth