Ground rent is a payment you make to your landlord as a condition of the lease. As with any rent, if you must pay ground rent this will be stated in your lease and you should pay it by the due date.
Although it is your responsibility to pay the ground rent, the landlord must give you notice of it using a form prescribed by the Landlord and Tenant (Notice of Rent)(Wales) Regulations 2005. Landlords cannot legally recover ground rent unless they have asked for it first.
The landlord must issue the demand for ground rent in the prescribed format in order to collect it. If you pay in advance or by monthly direct debit you do not need to contact the landlord, as Bron Afon have included this charge into your payment plan. You should just keep the demand for your own records.
All this information is included in your lease. The amount is fixed for the duration of all current Bron Afon leases. The each lease will provide a fixed date in the year and the dates vary in different leases. Some ground rents are annual and some are bi-annual, again this is determined in the lease.