Stuart
I'm assuming you've been tested for diabetes, hypertension, thyroid function, orbital fractures and stroke. Double vision can be caused by so many different things - I have 'divergence excess exophoria' which means that my eyes turn outwards; they are helped by a small prism in my glasses. My problem is congenital and hereditary - my aunt and sister both have it, as did my grandmother. If they can find no medical cause for the double vision (we optometrists call it diplopia), then try not to worry; get checked out by an optometrist.