For landlords looking to forfeit a lease, following a tenant default, it is important to ensure that any requirements in the lease to notify the tenant of the relevant breach and allow a period of time for the tenant to remedy the breach are followed before embarking on the formal forfeiture process.
Where a lease contains such notice requirements, the right for a landlord to forfeit the lease will not arise until the time period allowed to the tenant to remedy the breach expires and the tenant still remains in default. It is at this time that the section 146 notice (required under the Law of Property Act 1925) can be served by the landlord, which is the precursor to a court application for possession.
A recent High Court case, Toms v Ruberry  EWHC 2970 (QB), dealt with a situation where a landlord served the notice under the lease requiring the tenant to remedy breaches of repair within a certain period at the same time as the section 146 notice. The court dismissed the landlord's application for possession and found in favour of the tenant despite the fact the tenant remained in breach of certain repairing obligations.
Given the severe nature of forfeiture there is a clear statutory process which must be followed; the right to forfeit, and thus the obligation to serve the section 146 notice, does not arise until the tenant has failed to comply with a notice under the lease to remedy the breach.
Not all leases will have this additional notice requirement however and you should always check the terms of your specific lease carefully to ensure all pre-enforcement steps have been adequately followed.