Partly solved, the following bit works for an editable combo control
so I can now enter text to it:-
I found that by trapping the WM_LBUTTONDOWN to set the focus I could
then test not only the msg.hwnd to see if it was one of my controls
but, if that failed, test the result of ::GetParent(msg.hwnd) and if
that fails ::GetParent(::GetParent(msg.hwnd)) to look for a recognised
hwnd and then pass the recognised hwnd to IsDialogMessage() as normal.

The above solution does not work for the standard "edit" class of
control that I create with something like this: