Ok, I have just realised that if I change the below field on the form from a Checkbox to a Text field it works:
Dim nodes As XPathNodeIterator = _
root.Select("/dfs:myFields/dfs:dataFields/d:tblSite/d:tblAlarm/d:tblFreeStandingCashSafe/@SafeBoltMonitoring", NamespaceManager)
Any reason why the type of field will stop ...