I've just answered my own question.
You have to edit the landing page, then approve and close.
Magically that makes things work.
I had presumed that the form was simply embedded and therefore changes would flow through automagically. Apparently not!
Right, when you use a named Form element on an LP, the LP contains a secondary replica of the form that's not necessarily the same as the approved version of the form in Form Editor.
When you use embedded Marketo forms (either on non-Marketo pages or on Marketo-hosted LPs, though the latter is not recommended) then the approved form is used immediately, without a need to touch the LP as well.