TR19® Grease now states that the entire kitchen extract system must be cleaned from end to end and if it not this must be reported and clearly identified on a drawing to the client and their insurance provider.  A copy of this report should also be uploaded to the B&ES ventilation hygiene register.  We have an inhouse bespoke system to clean vertical risers from top to bottom using a combination of reuseable safety access systems that we install and tried and tested cleaning methods backed by client case studies and references.  There are may companies offering mechanical and chemical solutions for non-accessible ducting that damage the ducting and have limited effect when cleaning grease especially in ducting that is not round.  Just think ‘round peg - square hole’ and you will understand the limitations of mechanical solutions.