tenacy rights

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my business occupies rented accommodation,part of which is owned by a second party and is only accessible throught the other tenacy. The second party wishes to increase the rent on this portion. We have occupied both parts of this building for the past twelve years without a formal tenacy agreement or lease. The owner of the first part is talking about re-development which would mean us having to find alternative premises. What are the rights of both landlords and us as tenants?