Traditionally that was probably usually the case but, thee days, new owners (leasholders in this can) can, and often do, 'switch suppliers' the moment that take possession of a property.
The two surely go hand-in-hand. Electricity cannot be supplied if the equipment supplying it is not satisfactorily 'maintained'
I imagine that depends upon what it is that is 'potentially dangerous'. If it relates only to equipment supplying the particular leaseholder with electricity, then it's not necessarily obvious who else might be (partially or completely) 'responsibility, is it? In the case of multiple leasehold flats within a building, there will often be a "landlord's supply", for electricity used in 'common areas', and that [presumably complicates matters a bit?