The notice is invalid on the face of it, as it is not in the prescribed format required to end an assured shorthold tenancy agreement, which effectively means that the landlord has not in fact issued you a notice at all. If it had been served in the correct format, but on 21st September, it would also be invalid if your tenancy agreement stated that it couldn't be served until the term had ended - however this is unlikely as most tenancy agreements state that notice can be served to end ON the last day of the agreement, so he could serve that at month ten in a twelve month tenancy for example. Immaterial in any case as the notice is not in the correct format.
However, the issue seems to be more that the property is uninhabitable, and the landlord only wanted you to move temporarily whilst it was repaired. If your tenancy is actually over, then you are on a periodic tenancy agreement, which can be ended by proper service of notice, which in the circumstances, given the disrepair that the landlord needs to resolve, could be less than the two months referred to in your agreement if alternative accommodation has been offered.
Your landlord clearly hasn't taken any legal advice as yet, which is why he thinks he has served notice on you (when he hasn't), but when he does take advice, he'll discover he can serve notice on a mandatory ground as the property is in need of repair.
Your landlord has an obligation to keep the property in good repair and he is probably anxious for you to move so that he can ensure he is not breaching the law. You could ask him to pay your additional costs involved in leaving as compensation.
Does that help?