Upload files to SharePoint Online using REST API

I'm trying to develop a console application which will upload files from the users client PC to a SharePoint Online site.

I've written a version that used CSOM, but the file size is limited to 1.5Mb, so now I'm trying to use the REST API (it supports files up to 2Gb), however when I try to perform the upload all I get is 'forbidden'

I successfully obtain the form digest value, the code I have is

Public Shared Function UploadFile(baseURL As String, digest As CDigest, sourceFile As String) As Boolean

    Dim byteArray As Byte() = IO.File.ReadAllBytes(sourceFile)

    Dim folderName As String = "/TestFolder"
    Dim fileName As String = "Test.txt"
    Dim s As String = "/_api/web/GetFolderByServerRelativeUrl('[Folder]')/Files/add(url='[Filename]',overwrite=false)"
    s = s.Replace("[Folder]", folderName)
    s = s.Replace("[Filename]", fileName)

    Dim req As Net.HttpWebRequest = Net.HttpWebRequest.Create(baseURL & s)
    req.Method = "POST"
    req.Accept = "application/atom+xml"
    req.Headers.Add("X-RequestDigest", digest.Value)
    req.Headers.Add("binaryStringRequestBody", "true")
    req.Headers.Add("X-HTTP-Method", "PUT")

    req.ContentLength = byteArray.Length
    Dim ds As IO.Stream = req.GetRequestStream()

    ds.Write(byteArray, 0, byteArray.Length)

    ds.Close()

    Dim res As Net.HttpWebResponse = req.GetResponse() ' Errors here with Forbidden

    Console.WriteLine(CType(res, Net.HttpWebResponse).StatusDescription)

I would grateful for any assistance

Thanks

Lee...