[AccessD] A2003: Undo Changes to a subform

Hello all
This should be simple :-))

If I type me.undo and attach it to a button on a form I can undo edits to a record.

If I have a main form that has a sub form in it and if I attach code something like
Forms!MyForm!MySubForm.Form.Undo to a button on the main form with the intention of
Undo-ing any edit to the sub form it doesn't seem to work

How do I undo edits to a sub form from a button on the parent form??

Many thanks

Darren