Always try and work with the school, however if they are not putting in place the provisions recomended by professionals then you have no options than to look at the alternatives open to you to use.
I would say that you don't have to use the chair of governors of the school, the local government ombudsman or the director of education or even sendist as in saying to them that you "will" use these services, sometimes its another route to suggest that you "may" involve these services which may get them moving.
The reson you have to fight so hard is there is so little teacher training in this area and at the moments and so little funds for heads to send them on courses.
Teachers find in hard enough just doing the day to day and then deal with any extras on top. A bit like us really. So asking them to go the extra mile when they feel stretched to the limit is hard.
A head also can't provide a TA to assist the teacher without funding and they have to account for that funding say what they spent it on why and with what evidence did they base their descision on.
A head of a school or the senco might get criticised for not being able to spend their budget wisely. They are expected to spend the most on children who need the most support physically and academically, social communication comes the last in their list most often as most people in general have no idea of how disabling ASD is as their framework is from the more severe end of the spectrum usually were it can be more visible to them.
I think the tribunal let you down last time.
I also think that you need to feel strong enough to battle. I have to break down the tasks letter, writing and space out when I do them so its not all consuming.
I think give yourself a month of, keep a diary of anything school related though and then see how you feel about the need to go further or if you are happy with the way things are and to leave them at that?