Thanks. Found something a bit like it, changed it, and now have a routine to
do it. For those that want it:

// Draw a reflection of an image
public static void DrawReflection
(
   Graphics graphics, // the Graphics surface to paint on
   Image image,  // the Image to take a reflection of
   Color backgroundColor, // the background colour
   Rectangle destinationRectangle,  // where to paint it (should be under a
painted object and the same size as it)
   int reflectivity    // 90 is recommended, 255 is full reflectivity, 0 is
none (so don't use it!)
)
{
   // Crop the reflection to the bottom third (or depending on
reflectivity)
   int reflectionHeight = (image.Height * reflectivity) / 255;
   Image reflection = new Bitmap(image.Width, reflectionHeight);
   Graphics g = Graphics.FromImage(reflection);
   g.DrawImage(image, new Rectangle(0, 0, reflection.Width,
reflection.Height),
       0, image.Height - reflection.Height, reflection.Width,
reflection.Height, GraphicsUnit.Pixel);
   g.Dispose();

   // then flip it
   reflection.RotateFlip(RotateFlipType.RotateNoneFlipY);
   Rectangle imageRect = new Rectangle(destinationRectangle.X,
destinationRectangle.Y,
       destinationRectangle.Width, (destinationRectangle.Height *
reflectivity) / 255);

   // Place it on the window.
   graphics.DrawImage(reflection, imageRect);
   // Create gradient brush depending on reflectivity
   LinearGradientBrush brush = new LinearGradientBrush (imageRect,
          Color.FromArgb(255 - reflectivity, backgroundColor),
           backgroundColor,
           90, false);

   // And paint it over reflection image ...
   graphics.FillRectangle(brush, imageRect);
}