Generally leases contain forfeiture rights.
Forfeiture allows the landlord to terminate the lease in specific circumstances, e.g non payment of Rent or breach of any obligations in the lease.
If the lease contains this right you could potentially rely on it.
A tenant can obtain relief from forfeiture, in certain circumstances, by remedying the breaches.
Are you the landlord?
You granted a new lease to a tenant who then resold it, is that correct?
Have you checked title at the land registry to establish who the tenant is?
There should be a clause allowing you access on notice to inspect and then serve notice on the tenant if it is in breach. The lease should say how long the tenant has got to repair before you can enter and do the work in default and claim the costs off the tenant. It the tenant refuses to pay you sue him. This only really works on disrepair and failure to decorate.
With regard to the conversation to two flats, did the tenant get planning permission? You could report to the council. You could also report to his lender as he may have breached the terms of his mortgage.
You could take steps to forfeit the lease on grounds of breach of repair and decorating obligations and creating two flats. There is a specific procedure to follow initiated by serving notice on the tenant. The tenant can, in certain circumstances claim relief from forfeiture. If the tenant has a mortgage the lender can obtain relief also.
Your success in following this process will depend on the circumstances and how long you have been aware of the tenant's breaches and your previous attempts to remedy.
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