I’ve read somewhere that if you haven’t paid your ground rent for 12 years you don’t have to worry after that time.
You’re probably thinking of squatters rights or adverse possession whereby you may be able to claim ownership of a piece of land if you have used it uninterruptedly for a number of years without asking permission or without being challenged. Unfortunately you do have permission to use the land, in the form of a lease. These days you are not liable to pay ground rent unless you receive a demand for it, and many people find themselves in your situation because it’s not cost effective for landlords to chase small amounts of money, however you should familiarise with the terms of your lease and stick to them.