Google "how to do a COSHH assessment" Lots of info there. I blitzed all mine ages ago. The really irritating thing is when you have several products with the same stuff e.g. glyphosate as each product needs its own assessment. Thank God for copy and paste!
Remember you should have a COSHH assessment for things that may seem innocuous, such as hydrated lime, obviously cement, FEB mix and things that are created on the farm or in the environment, maybe Leptospirosis from cattle or dust from grain dryer/cleaner, and I guess oils, paints, waste oil from tractors. I think if we are being really anal about it, each assessment should have an accompanying hazard data sheet from the manufacturer that has a current'ish date on it and the assessment should be re visited to make sure it is still current periodically (I would guess that is every year or at most two). Suppliers have a duty to give you the hazard data sheet for stuff they sell to you. I may be inclined just to collate the hazard data sheets and see if that is adequate for your needs?