Extending a lease – Formal or informal approach?

I am seeking to extend the lease on my flat (currently 91/92 years) by adding 90 years. Last year, I contacted the freeholder informally and negotiated a reasonable premium directly with him without involving any surveyors or solicitors.

I then pressed paused on going ahead with it as I was concerned about risks associated with the freeholder changing his mind, such as requesting changes or premium amount later down the line when I have incurred legal costs. I am happy with the premium but do not want to expose myself to the risks.

I would welcome the forum’s views and experiences on extending leases and how best to navigate this situation.

How concerned should I be if I take the informal approach and are there any other risks with the informal approach? Is there a hybrid approach where I adopt a formal approach but pre-agree a premium so surveyors are not involved?

Thanks,

Max