The landlord is not under a duty to volunteer the fact that he knew about the development (if he did), but if asked about it then he would have to tell the truth. A direct question ("are you aware of plans to develop") might enable you to terminate, or an indirect one ("should we anticipate any disturbances from neighbours or surrounding areas") might also catch him.
The distrubances conversation in the above form would not quite get him on the hook in my view. You would also have to prove that he knew the property would be developed.
If you could prove this misrepresentation then you could terminate on the basis of this (repudiatory) breach of contract. If you do this now he would no doubt dispute, sue you for the rent and you would have to litigate on shaky grounds. This presumably would not be in the best interests of your young famiy.
This is not to say you cannot try and get out of it informally in corresponding with the landlord. It costs nothing to sabre-rattle and there is no harm in overplaying your position. See how that goies and if he knows that you are bound and confident he did not misrepresent himself then you will have to see what you can negoaitate with the landlord if the development starts. Perhaps a reduced rent or surrender (where you agree with him that the lease should be mutually termianted) where you pay him a sum of money to give up the lease.
Landlords are under a general duty not to refuse a reasonable repacement tenant and you should also focus on finding one to take over the tenancy. www.gumtree.co.uk is quite good for this.
Sorry it could not be better news.
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