WidthRequest / HeightRequest not being honoured on images in a stacklayout -> ViewCell -> ListView

I'm trying to fudge together an image grid. Given that there isn't a bindable grid control at the minute, our "fudged" solution was to come up with a simple wrapper model to display 3 sequential images in a row within a ListView.

But I'm having some trouble getting the images to display in the dimensions I want.

Each image in the collection is a landscape 400x300px Jpeg.

Below is the code for the 3 images within the row template. No matter what I do for the WidthRequest/HeightRequest, I cannot get it to display as a square thumbnail image. I've tried tweaking the Aspect to all 3 values (AFit, AFill & Fill) but the image will always be 400px wide.

Any idea what I'm doing wrong ?

ThumbnailList.ItemTemplate = new DataTemplate(
            () =>
            {
                var device = Resolver.Resolve<IDevice>();
                var width = 640; //assume 640px min display default
                if (device != null && device.Display != null)
                {
                    width = device.Display.Width; //returns 720 for a samsung s3
                }
                var thumbwidth = ((width / 10) * 3); //216 px on 720 wide.

                Image imgCol1 = new Image();
                imgCol1.WidthRequest = thumbwidth;
                imgCol1.HeightRequest = thumbwidth;
                imgCol1.Aspect = Aspect.AspectFit;
                imgCol1.HorizontalOptions = LayoutOptions.Start;
                imgCol1.VerticalOptions = LayoutOptions.Start;
                imgCol1.SetBinding(Image.SourceProperty, "UIPhotoCol1.Source");
                var commandImg1 = new TapGestureRecognizer()
                {
                    Command = viewModel.DeletePhoto,
                    NumberOfTapsRequired = 1
                };
                commandImg1.SetBinding(TapGestureRecognizer.CommandParameterProperty, "UIPhotoCol1.InternalTrackingId");
                imgCol1.GestureRecognizers.Add(commandImg1);

                //Repeat block for imgCol2
                //Repeat block for imgCol3

               return new ViewCell
                {
                    View =
                        new StackLayout
                        {
                            HorizontalOptions = LayoutOptions.Start,
                            //Padding = new Thickness(0, 5),
                            Orientation = StackOrientation.Horizontal,
                            //Spacing = 5,
                            Children = { imgCol1, imgCol2, imgCol3 }
                        }
                };
            });