Hi, Thank you for your question and welcome. My name is ***** ***** I will assist you. If you are not receiving an invoice you do not have any obligation to hunt the freeholder down and try and pay the ground rent. They will likely become statute barred from claiming any ground rent that becomes older than 6 years and is not claimed.
The land the houses is on, the freehold, likely belongs to the freeholder's estate (if they passed away). You can find out who the owner is at the Land Registry
As you have lived their for more than 18 months, if you can get the other flat owners to agree, you may be able to force the landlord to sell you the freehold
I look forward to hearing from you. Kind regards AJ