I'm a MN regular and also a SENCo and primary teacher. I'm studying for an MA in Special and Inclusive Education at present and considering dissertation topics. In my experience, children with speech and language difficulties find it difficult to learn to read. This is not a statement I can support scientifically, it is just my experience that most of the pupils in my school who are significantly behind in their literacy skills also have significant speech and language difficulties eg a stammer or problems with speech sounds.
I'd be grateful to hear from anyone with a child with Sp&L difficulties who can support or reject my theory that the two are related, thank you!