UK Landlord Forum / Life

Hi, we own the freehold to a building with two flats in it. We own the bottom one and let it out. The flat upstars is owned as a leasehold. we do not charge the lease holder above any ground rent or a service charge. I understand we cannot start to charge ground rent but Can we start to charge them a service charge monthly? and if so how do we go about doing it?

What does their lease say?