I had the same frustration myself and so decided to train my SLT on the sly! First I created an overview document based on the code of practice which outlined the rules and responsibilities for SENCZo, teachers, SLT and governors. Next I created a set of expectations training session for all staff in which I related the way in which SLT would support me and how I would support them! I have also written a document which outlines how my role overlaps and coincides with so many other parts of school life.
I find this aspect of my role more infuriating than any other… it’s no wonder it’s recommended we are part of SLT!