Hill & Redman's Law of Landlord and Tenant 16th Edition page 557 paragraph 446 has this to say on the proof of breach of covenant against assignment, underletting or parting with possession:-
"405. Covenant against assignment or parting with possession – covenant "not to assign" (a) or "not to assign or otherwise part with" (b), the premises is only broken by a legal assignment for the entire residue of the term. Consequently the covenant is not broken by a declaration of trust of the premises in favour of a third person (c), or by the deposit of the lease as security for an advance (d), or by the grant of an under lease of part of the premises (e) or for part of the term (f). A covenant "not to assign or part with the possession of premises" is often inserted in order to prevent a tenant from evading the restriction against alienation, by putting another in possession without either an assignment or an under-lease. "Parting with possession" means to any person other than the person to whom possession was given by the original lease. Such a covenant is broken if the lessee makes an equitable assignment of the lease and places the assignee in possession, but a lessee who retains the legal possession of the whole of the premises at all material times does not commit a breach of the covenant by allowing other people to use the premises (g). The retention of possession by one partner alone on a dissolution of the partnership is not a breach of a covenant against assignment contained in a lease to both (h); but if one executes a formal assignment to the other this is a breach (i). A covenant not to part with the possession of the premises is not broken by the lessee parting with part of the premises (k).
(Italics mine).
The law is that the burden of proof of the breach of the covenant alleged remains on the plaintiff in an action for forfeiture based on the breach of that covenant. In an action for forfeiture in Duke's Court Estates Ltd. v. Associated British Engineering Ltd. (1948) 2 All E.L.R. 137 at page 139, Harman J. said on the issue of burden of proof of forfeiture as follows:-
"It seems to me that prima facie – although I am only deciding this so far as it is necessary for the purpose of my decision – the burden in a forfeiture action (as is shown by the two cases cited to me by counsel for the defendants, Doe d. Bridger v. Whitehead (1) and Toleman v. Portbury (2), is particularly laid on the plaintiff, and here the plaintiffs, in order to deprive the defendants of the estate which the plaintiffs have granted to them, must show that the defendants have permitted user by persons who are not associated with them in the various aspects of their business. It does not seem to me enough for them to come to the court and say: "The defendants admit that they have permitted these people to occupy part of the premises and, therefore, it is for them to prove that they are within the privileged class." The boot is on the other foot. If the action depended on the covenant not to assign or underlet, the matter might be quite different. If the defendants then wished to confess and avoid by alleging that they had assigned or underlet, but had been permitted to do so, the burden would clearly be on them.
I come to the conclusion that the burden remains on the plaintiffs in spite of the admissions made in the defence."
Where, as it is the case in this appeal, no time limit was fixed for the development to be executed, the particulars of the alleged breach of the covenant will consist of (a) the lessor's opinion as to the time within which the covenant to develop should take place and his reasons for so holding (b) the fact that such a period of time has passed, and (c) that the development has not taken place.