As this is from the main form & only one control is on the subform then it should be
Code:
Private Sub Form_Current()
Me.[Cog signatory the same?]=Me.[2017-18 data subform1].[Form]![Full name of person who agrees to the terms of the Conditions_01]=Me.[name on form]
End Sub
Or
Code:
Private Sub Form_Current()
If Me.[2017-18 data subform1].[Form]![Full name of person who agrees to the terms of the Conditions_01]=Me.[name on form] Then Me.[Cog signatory the same?]=True
End Sub
Or follow June's suggestions
BUT PLEASE RENAME YOUR FIELDS, FORMS, CONTROLS & LABELS .....