UnProject method

I need an OpenTK equivalent of gluUnproject in order to get 3D coordinates at the mouse position. I have cobbled this together (adapted from a couple of sources):

Vector3 v = GetOGLPos(e.X, e.Y);

Vector3 GetOGLPos(int x, int y)

        Matrix4 projection = new Matrix4();
        Matrix4 modelview = new Matrix4();
        Size viewport = new Size(glControl1.Width, glControl1.Height);
        float winX=0, winY=0, winZ=0;
        GL.GetFloat(GetPName.ModelviewMatrix, out modelview);
        GL.GetFloat(GetPName.ProjectionMatrix, out projection);
        winX = (float)x;
        winY = viewport.Height - (float)y;
        GL.ReadPixels(x, (int)winY, 1, 1,PixelFormat.DepthComponent, PixelType.Float, ref winZ);
        Vector4 vec = UnProject(ref modelview, ref projection, ref viewport, new Vector3(winX,winY,winZ));
        return new Vector3(vec.X, vec.Y, vec.Z);

private static Vector4 UnProject(ref Matrix4 projection, ref Matrix4 view, ref Size viewport, Vector3 mouse)
Vector4 vec;

        vec.X = 2f * mouse.X / (float)viewport.Width - 1;
        vec.Y = -(2f * mouse.Y / (float)viewport.Height - 1);
        vec.Z = 2f * mouse.Z - 1f;
        vec.W = 1f;
        Matrix4 viewInv = Matrix4.Invert(view);
        Matrix4 projInv = Matrix4.Invert(projection);
        Vector4.Transform(ref vec, ref projInv, out vec);
        Vector4.Transform(ref vec, ref viewInv, out vec);
        if (vec.W > float.Epsilon || vec.W < float.Epsilon)
            vec.X /= vec.W;
            vec.Y /= vec.W;
            vec.Z /= vec.W;
        return vec;

It’s not working. It does something, but z is always 1. Can anyone help?