VB.net - Getting Data from an XML File

I am trying to write a small program to get some data from an XML file which is already being used by another application (NOT MY OWN).

The XML Looks like this ...

?xml version="1.0" encoding="utf-8"?

sunjournal

rvcmappings default="MISSING"

  rvcmap unitId="2" rvcnum="443" /  

  rvcmap unitId="3" rvcnum="103" /

  rvcmap unitId="5" rvcnum="701" /              

/rvcmappings

/sunjournal

I am trying to use the below Code in VB to get the "rvcnum" for the UnitId of 5.

Dim doc As XmlDocument = New XmlDocument()
        doc.Load("C:\BootDrv\Aloha\RptExport\GLMapping_Master.xml") 
Dim acc As String = doc.SelectSingleNode("sunjournal/rvcmappings/rvcmap[UNitId='5']/rvcnum").InnerText

msgbox(acc)

Can anyway point me in the right direction as I do not receive any Errors in Runtime just doesn't show the any Msgbox Data??

Many Thanks Rob