Complaints should be raised with the office holder in the first instance. At Agri Assist that's Rob Sadler who may be contacted directly at firstname.lastname@example.org.
If you are still not satisfied, a complaint can be made to the Recognised Professional Body of the office holder, or via the Complaints Gateway, managed by the Insolvency Service.
Rob Sadler's Recognised Professional Body is the Insolvency Practitioners Association. Their complaints policy can be accessed at the IPA website.