''As there is, at the present time, no infallible way of separating the 'dyslexics' out as a special group, either ALL poor readers are dyslexic (correct, in the descriptive sense that they all have difficulties with the written word), or none are"
That is a matter of opinion, not fact, and the very argument that was put forward in the controversial documentary I mentioned in my previous post. Dyslexia is actually a much wider issue than "poor reading". If someone is simply a 'poor reader' but does not have the key characteristics of dyslexia - such as, a poor working memory, directional confusion, struggles to get thoughts onto paper or takes longer than average to do so, inconsistent performance across different areas (i.e. performs very well with certain tasks but struggles with others that most people tend to find easy - hence the relevance of discrepancies in IQ scores), then a clear distinction between 'poor readers' and dyslexics can be made.
In response to BeingFluffy's question, the exact cause of dyslexia is still not known. Much of the evidence points to a heriditary component and brain differences have been identified.
In response to questions about where to get your child asssesed, I recommend Dr David McLoughlin (Chartered Psychologist) at Independent Dyslexia Consultants. I paid 400 for my daughter's assessment (about a year ago). I believe that Dyslexia Action refer people to many different Psychologists and charge about a hundred pounds for making the referral.