Just because a lease stipulates a service charge do not make the assumption that the tenant must pay it in advance. Yes, the vast majority of leases you encounter will state, clearly, that the service charge is an advance payment by the leaseholder. However some leases, mainly older ones, will instead provide a mechanism whereby payments are made in arrears. This has a profound impact on how you will manage cashflow and is therefore a vital piece of information to know and understand.