VBA interface with MapPoint (datatypes)

What I'm trying to do is have Excel interface with MapPoint (v17 - 2010) in order to automatically work out drive times between two grid references. However, while the code below works perfectly if I have 2 postcodes, if I try it with grid refs instead it gives me the error: "The requested member of the collection does not exist. Use a valid name or index number".

    Dim objApp As New MapPoint.Application
    Dim objMap As MapPoint.Map
    Dim objRoute As MapPoint.Route
   
    Set objMap = objApp.NewMap
    Set objRoute = objMap.ActiveRoute
    objApp.Visible = True
    objApp.UserControl = True
    

    szZip1 = Range("e5").Value
    szZip2 = Range("I5").Value
    

  'Add route stops and calculate the route
  objRoute.Waypoints.Add objMap.FindResults(szZip1).Item(1)
  objRoute.Waypoints.Add objMap.FindResults(szZip2).Item(1)
  objRoute.Calculate
    objMap.Saved = True
  Range("L1").Value = CStr(objRoute.DrivingTime / geoOneMinute)

The problem is I only have one postcode, the second data source is worked out from looking up the northing and easting (grid ref) of a number of postcodes from a database I have (yep, of 1.7 million postcodes and their grid refs) and taking the average.....but I have no way to convert back the other way.

So.....I either need someone to tell me how I can get the above code working with grid refs instead of postcodes or a way to get the nearest postcode from a grid ref. Any ideas?

Came up with a plan to reverse look up grid refs against the nearest valid postcode......but I'll leave the thread up in case anyone does know anything about interfacing with MapPoint cos any info would be handy.

Hugely helpful, thanks Eric.