Thanks to Dan 's comment I can now report that I have at last, seen the cow. Every time I look at the picture now I see the cow. Its not a necker cube thing where the picture oscillates between two possible interpretations. The cow image is the only viable interpretation really.
I think the crux of my problem was that I was trying to treat the black and white bits as outline. If you treat the black and white as tonal variation (squinting is recommended) I think you will find the cow easier to see.
I cannot tell you for sure because, now that I can see the cow, my brain has changed shape slightly and I have no way to revert it.