Undertaking the risk assessment doesn't give you the answer - it assesses the problem in a structured manner.
You need to urgently familiarise yourself with the Working at Height Regulations 2005. They are here.
http://www.hse.gov.uk/pubns/indg401.pdf
That will tell you what the minimum safety guard system you must put in place.
But in summary you either need a suitable guarding system to prevent a fall OR a harness system. You don't need to do both - but having both reduces the potential for accident even further. Risk is not a 'digital' thing (an activity is never either 'safe' or 'risky').