The motorbike is a hazaard and should not be permitted. Not only is it the potential fire risk as you say, but if people are directed down that corridor, possibly in the dark in the event of an incident, it impedes their way - all routes to fire exits must be kept clear. Fire Regulations are statutory, legal requirements, and must be adhered to.
This is very basic H&S - I am not sure why you have been asked to do this risk assessment. If H&S is deemed to be your responsibility then your employers really should be sending you on accredited risk assessment training before they ask you to do so.