In some measure of progress since my last question I have got the following project to compile. It is based on https://stackoverflow.com/a/10949784/846550
However at runtime the call to CoCreateInstance fails. The HRESULT is 0x80131522 - google for this and most hits seem to relate to SQL server which I am not using.
C++ code, unmanaged
#include <iostream>
#include <atlbase.h>
#import "..\ClassLibrary1\bin\debug\ClassLibrary1.tlb" no_namespace
using namespace std;
int _tmain(int argc, _TCHAR* argv[])
{
    ATLENSURE_SUCCEEDED(CoInitialize(NULL));
    CLSID clsid;
    _ComClass1 *t; 
    ATLENSURE_SUCCEEDED(::CoInitialize(NULL));
    ATLENSURE_SUCCEEDED(CLSIDFromProgID(OLESTR("ClassLibrary1.ComClass1"), &clsid));
    HRESULT hr = CoCreateInstance(clsid,NULL,CLSCTX_INPROC_SERVER,
                                   __uuidof(_ComClass1),(LPVOID*) &t);
    ATLENSURE_SUCCEEDED(hr);
    ATLENSURE_SUCCEEDED(t->showform());
    ATLENSURE_SUCCEEDED(t->increment());
    CoUninitialize();
    return 0;
}
Here is the DLL code for good measure - almost completely autogenerated as a COM DLL class in VB. (The DLL shows up as registered using oleview as well).
<ComClass(ComClass1.ClassId, ComClass1.InterfaceId, ComClass1.EventsId)> _
Public Class ComClass1
#Region "COM GUIDs"
    ' These  GUIDs provide the COM identity for this class 
    ' and its COM interfaces. If you change them, existing 
    ' clients will no longer be able to access the class.
    Public Const ClassId As String = "46783a4a-5ca9-4930-addb-cde91a377c02"
    Public Const InterfaceId As String = "e2ad7298-7339-45fb-aad1-cb82188bf067"
    Public Const EventsId As String = "b7e10cdf-dc2a-4052-9b0f-f6f9d5a1f0ac"
#End Region
    ' A creatable COM class must have a Public Sub New() 
    ' with no parameters, otherwise, the class will not be 
    ' registered in the COM registry and cannot be created 
    ' via CreateObject.
    Public Sub New()
        MyBase.New()
    End Sub
    Private f1 As Form1
    Public Sub showform()
        f1 = New Form1()
        f1.Show()
    End Sub
    Public Sub increment()
        f1.ProgressBar1.Increment(5)
    End Sub
End Class