Hi alfie, I reckon your best change of getting a proper understanding of how behaviour management works at the school is to get a spread of opinions from parents who have kids there.
If it's an indie day pupil SS, then I would guess that a fair number of parents would be pretty clued up and engaged with behavioural management issues and won't be fobbed off by a fancy Ofsted-complying jazz hands policy document Most parents with kids at indie SS will have thought long and hard about the placement, and will have had to fight someone to secure it - and sometimes, they will have had to fight to improve it. For my money, that usually makes them useful people to talk to. Definitely speak to several parents if you can though, just in case you get someone with an axe to grind or a friendship with the HT to protect.
If you don't know any parents who have kids there already, then ask the school if you can speak to some parents - if the school is reluctant to do this, then that in itself tells you something useful....
The Ofsted grade issue wouldn't worry me in the slightest - the quality of inspectors covering special schools varies massively, as you can tell from Agnes and ourvye's posts. It takes laughably little experience to be able to brand yourself as a 'specialist' Ofsted inspector - in most cases, you will have more worthwhile experience of assessing the quality of support for your kid's SEN than the Ofsted inspector who is trousering £300-500 per day for the privilege.
The last few Ofsted reports for your indie SS should name the inspectors on the first or second page. You can check to see whether these inspectors actually have any relevant and worthwhile professional expertise by clicking on the pen portraits at this link
Usually, if it's an HMI directly employed by Ofsted, they'll have some useful expertise. If it's an Additional Inspector (AI) employed by a private subcontractor, the chances are they won't have. Watch out for the clowns who describe themselves as a "specialist in SEN" - no-one with genuinely relevant experience in meeting your child's particular educational need would describe themselves that way. The "specialist in SEN" who inspected my DS1's provision turned out to have been a mainstream class teacher who could remember teaching one child with ASD and another child with moderate deafness when she last taught. Which was 15 years ago.