Lurked Actually chandler you're incorrect, not adhering to a clause of a contract, whether tenancy or not, doesn't automatically result in a court finding it a breach. Contract law even at a very basic level allows a court to make exceptions for a range of reasons. I'm too lazy to bother googling and I'm sure you're capable of doing it yourself if you don't believe me, but I bet the information is easily found to prove it.
That's not what I said. A breach of contract can potentially end a contract. Obviously not all breaches will end a contract. This is very basic contract law - generally, in scots law material breaches will end a contract (or if the contract was voidable in the first place due to error over the persons involved), in English law, it will depend whether it is a breach of a term or a condition. Personally, I would state whether a term is material or not, or that all terms are material, so as to reduce the court's freedom to decide that for me. Its a matter of good drafting.
Perhaps you should, if you are interested, and not "too lazy", quote the English version of the statutory grounds that is equivalent to Ground 13. If you deal with leases, it will be easy enough to find as it will be appended to the lease.
And of course if you specify those entitled to reside in a property, it would in many cases be a breach of contract if others were moved in. Of course landlords offer different prices based on how many people will be living there! I would think that, especially in this market, if you have a smaller property and are struggling to find a tenant, and one person comes along and says "its only for me, can you offer me a lower rent because I would be struggling on my own to pay more, and it will only mean half or less wear on the property" is quite a common scenario.
I think you are trying to allude to the concept of reasonableness under s. 18 of the Unfair Contract Terms Act 1977 and I agree that a baby or two would not void a contract of lease, but that several other adults or perhaps even one more adult, depending on the terms of the contract, could. In the OP, its clear the landlord is anxious because the tenant is already on benefits and living in a small property with two children, and appears not to show concern for the children living in such conditions in the future.