Once I run
Dim Old_User_ctrl As Control
Old_User_ctrl = Nothing
I break the code and using the immediate window I can still get to the control eg.
and later in the code I get the same error as before...
Multiple controls with the same ID 'MyControl1' were found.
But - I have worked around my problem now using a different technique. I have basically reset all of the existing control's variables, child controls etc. So I can use it like it was a fresh control.
So, I've solved my immediate problem, but if anyone can answer my original question - that'd be great for future reference !