The clerk seems to be doing exactly what I have written in the Q&As:
I cannot see that they have given any legitimate reason under the Data Protection Act for refusing to comply with your request.
We have only ever had one instance on here where the clerk and school refused to hand over the clerk's notes, and had an apparently legitimate reason (the issue was the way in which the notes were stored - the school completely shifted its ground from a ridiculous claim that Part IV Section 30 of the Data Protection Act applied, and started claiming instead that the notes were not part of an accessible filing system).
The forum member was Zeinab, and we subsequently obtained the notes by other means.
If you want to pursue this further, I suggest you telephone the ICO (Information Commissioner's) helpline for advice.
https://ico.org.uk/global/contact-us/helpline/Coincidentally, the General Data Protection Regulation (GDPR) comes into effect today, 25th May 2018, superseding the Data Protection Act. If anything, it strengthens your rights.
One difference is that Subject Access Requests under the Data Protection Act used to cost £10, but now they will normally be free of charge!
Unfortunately the introduction of GDPR means that the ICO helpline will be inundated with calls, so they are likely to be exceptionally busy.
It could be useful to see the clerk's notes before making a complaint because they are considered to be the authoritative record of your hearing.
It's just possible that mistakes have crept into the decision letter you received, and that the letter is not an accurate reflection of the notes.
There is also the possibility that the clerk's notes might reveal things that are not in the decision letter, which could strengthen the grounds for a complaint.