Think I have it sorted. Works so far. Thanks for all your help everyone.

Private Sub Document_ContentControlOnexit(ByVal aCC As ContentControl, Cancel As Boolean)
Dim aCC2 As ContentControl, sTag As String
Select Case aCC.Title


Case "C1"
If aCC.Type = wdContentControlCheckBox Then
sTag = aCC.Tag
Set aCC2 = ActiveDocument.SelectContentControlsByTitle(sTag)( 1)
With aCC2
If aCC.Checked Then
aCC2.Range.Text = "Title" & Chr(10) & "First line"
aCC2.Range.Words(1).Bold = True
Else
aCC2.SetPlaceholderText Text:="Blank"
aCC2.Range.Text = ""
End If
End With
End If
End Select
Select Case aCC.Title
Case "C2"
If aCC.Type = wdContentControlCheckBox Then
sTag = aCC.Tag
Set aCC2 = ActiveDocument.SelectContentControlsByTitle(sTag)( 1)
With aCC2
If aCC.Checked Then
aCC2.Range.Text = "Title1" & Chr(10) & "First line1"
aCC2.Range.Words(1).Bold = True
Else
aCC2.SetPlaceholderText Text:="Blank"
aCC2.Range.Text = ""
End If
End With
End If
End Select
End Sub

The project is almost complete now, thank you for all your suggestions, using content control definitely streamlined the process.

I have spent much time on research but hit a brick wall again. I am sure there were much efficient ways around it but the word document works well and checkboxes populate text in rich text areas so I would be reluctant to make any significant changes to the code, however the text needs formatting.

Unfortunately, one paragraph is over two pages long and should have different levels of lists formatting (Bullet points (chr(149), sub a) level and sub-sub i) level. Is there a way to select a range within the text and apply the list format? I am able to apply one list format to a whole range but not for parts of range and different levels of lists (as per above). Any help would be greatly appreciated again.