You appear to have some confusion with your syntax, and while I can't test it without access to the two documents the following should be a deal closer to what you require
Code:
Private Sub CommandButton1_Click()
Dim DocSrc As Document, DocTgt As Document
Dim RngSrc As Range, RngTgt As Range
Set DocTgt = ActiveDocument
Set DocSrc = Documents.Open("C:\Users\reception.AIRDG\Documents \Custom Office Templates\2019 Master Specification - Kate.dotm")
If CheckBox1.Value = True Then
Set RngSrc = DocSrc.Bookmarks("Demo").Range
Set RngTgt = DocTgt.Bookmarks("spec").Range
RngTgt.FormattedText = RngSrc.FormattedText
RngTgt.Bookmarks.Add "spec"
End If
If CheckBox2.Value = True Then
Set RngSrc = DocSrc.Bookmarks("Surveys").Range
Set RngTgt = DocTgt.Bookmarks("surveys").Range
RngTgt.FormattedText = RngSrc.FormattedText
RngTgt.Bookmarks.Add "surveys"
End If
DocTgt.Range.Fields.Update
Application.ScreenUpdating = True
Set DocSrc = Nothing
Set DocTgt = Nothing
Set RngSrc = Nothing
Set RngTgt = Nothing
End Sub