How do I start formal negotiations with my landlord?

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As this is a legal transaction, you should first get independent professional advice (legal and/or valuation advice), as you would have done when you acquired your leasehold property.

Whether you use the team who did your original conveyance or you want to appoint a new team, useful guidance is available from independent resources, such as the Leasehold Advisory Service (http://www.lease-advice.org).