I'm generally the first to object to silly jargon, but "differentiation" is about more than a two-way split in the way you teach as there are more than two categories of brains in the classroom, generally speaking. So it's not about "thick" and "brilliant", although it would be nice to think it was all that easy.
But why has it taken so many years for this thread to appear on Mudcat? Ofsted, sadly, has been around for quite a while now and there are quite a few of us who have suffered the slings and arrows of outrageous inspections ....