Why dont you try a softer approach and sit down with them and have a chat - tell them you intend to sell in the Autumn when the lease expires and you will give them the official notice in September, but you wanted to let them know now so they could start looking and find the right place. Maybe casually drop in that if they find somewhere sooner and want to come back to you that you are open to that, tell them they wont forfeit their deposit if they decide to break the fixed term lease, that you are happy to come to some sort of arrangement, decide before you speak to them what you want, you cant come back and claim foul if they leave within 2 weeks and want their deposit if you didnt discuss that with them, be clear about your expectations. Ultimately though they dont have to leave until the lease is up (and the required, properly serviced notice, notice period)
There is no rule that you cant give them more than the minimum notice set out in the act.
Without knowing the full lease and the full clause I can only speak in general terms, you cant break a fixed term lease, even if you wish to sell, the clause you mention could be referring to the renewal of the fixed term - ie to give 1 months notice of their intentions, or could refer to notice periods expected after the fixed term while in a Part 4 Tenancy, the link below will give you more info.
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