You could put your header merge field in a richtext CC titled "Ref"
Code:
Sub ScratchMacro()
'A basic Word macro coded by Greg Maxey
Dim oSec As Section
Dim oCC As ContentControl, oCC2 As ContentControl
For Each oSec In ActiveDocument.Sections
Set oCC = oSec.Headers(wdHeaderFooterPrimary).Range.ContentControls(1)
For Each oCC2 In oSec.Range.ContentControls
If oCC2.Title = "ref_no" Then
oCC2.Range.Text = oCC.Range.Text
'If there is only one "ref_no" CC per section then:
'Exit For
End If
Next
Next
lbl_Exit:
Exit Sub
End Sub