Hi Trevor,
Removing the Selected Item Transformation altogether makes no difference. HOWEVER ... I've stumbled upon the problem!
-- the asp.net Required Field validators!!! --
I had field validators to check required as well as valid email entry. The whole (small) form is displayed in a collapsible panel. Testing on a page with only 3 or 4 items listed, I opened all the panels, and could then easily see that submitting from only one of the forms triggered the validators for all of the forms! Just as an experiment, I entered data into all of the forms, and then submitted only one of them. Whalla! The form where I had clicked the submit button was the one that was actually submitted.
Will scrap the asp.net field validators and go for either some javascript, or just do the checking in the Submit code.