There are two types of breach of covenant that are generally considered incapable of remedy: 


not to assign, underlet or part with possession;


illegal or immoral use.


It might be thought that if a breach of covenant is not remediable then L would automatically be able to forfeit the lease. But, that is not correct; the fact that a breach is irremediable does not necessarily mean that T will be unable to obtain relief from forfeiture. This is illustrated by a recent case in which T was wrongly found to have shared possession, with the premises also being used for illegal and immoral purposes. On the facts, it was held to be appropriate that T should get forfeiture from relief (although he did have to pay L’s costs on an indemnity basis). Needless to say, such cases are exceptional but the point to appreciate is that the existence of an irremediable breach does not mean that L can forfeit the lease. See Patel & Anor v K&J Restaurants & Anor [2010] EWCA Civ 1211 (access free at www.practicalconveyancing.co.uk).