i'd get some kind of idea as to when he's likely to be "finished with it" if i were you. otherwise you could find that you have given him a tenancy of some kind. the fact that you may not have the right to give the tenancy (in legal terms) as the cupboard is either a "common area" or an area informally shared between 2 or more flats may not be relevant if he claims he has been using it for a certain period and makes a claim for adverse possession

be careful. you can check if the power the cupboard uses is yours by turning your ccu off and then going down to the cupboard and switching the light on. may be run off the landlord's power supply if it's a shared cupboard, although builders have been known to take short-cuts...