As the header says, I want to tab through my main form, into a subform, through the subform (no problem all done using the tab order), and then back into the main form at the next field in the tab order, any ideas.

What I do in this situation is to add code on the On Exit of a field in the subform (Your choice) That if that field is Empty then I set the focus to the next control in the Form.

Forms![frmMain].[NextControlName].Setfocus