Sorry I just read the thread and see you already filed a complaint.  Great.  Has it been acknowledged? 

My only suggestion here is to send new simplified FOIs .  your initial ask is quite extensive, and would require someone to compile quite a bit of information.  

For example, you may want to start by identifying who were the authors of that specific section of the risk assessment.  Then, you may want to FOI those individuals emails pertaining to the national risk assessment, or which contain the key words you want.  Maybe do so in a 3-4 month period.

This does require fishing blindly.  But try and dig down to the human(s) behind the statements and the work, and press them