Hi @ben_saundersHDKKB. As a quick test, I created a simple new GeneralNote in one of my drawings, then injected a link to the drawing's Part Number, " - ", a link to the drawing's Stock Number, " - ", and a link to the Sheet Number, similar to how the TextBox in your TitleBlockDefinition is set-up. Then I wrote a small iLogic rule which allowed me to 'Pick' that GeneralNote, and replace the <Sheet Number> portion of its FormattedText, with a "0" (zero). It worked just fine.
Dim oObj = ThisApplication.CommandManager.Pick(SelectionFilterEnum.kDrawingNoteFilter, "Pick a GeneralNote.")
If oObj Is Nothing OrElse (TypeOf oObj Is Inventor.GeneralNote = False) Then Return
Dim oGN As Inventor.GeneralNote = oObj
'Logger.Info("Text = " & oGN.Text & vbCrLf & "FormattedText = " & oGN.FormattedText)
Dim sTextToFind As String = "<DerivedProperty DerivedID='29704'>Sheet Number</DerivedProperty>"
If oGN.FormattedText.Contains(sTextToFind) Then
Logger.Info("Contains the text we are looking for = True")
oGN.FormattedText = oGN.FormattedText.Replace(sTextToFind, "0")
Else
Logger.Info("Contains the text we are looking for = False")
End If
However, keep in mind that in your case, you would be editing the 'definition' of your TitleBlock, which may effect the TitleBlocks on the other sheets, whose TitleBlocks may be based on the same TitleBlockDefinition. That is why I suggest at least having a copy of your original TitleBlockDefinition available, incase editing the contents of the TextBox changes the definition, or maybe just creating an alternate TitleBlockDefinition the way you want it for those types of sheets.
Wesley Crihfield

(Not an Autodesk Employee)