Thanks for your question.
The postilion is that if you were to simply leave the flat and not stop paying rent then the landlord would sue you for the rent outstanding until 1) a break clause or 2) the end of the term. He would probably get judgement at Court on the amount you would have paid in rent up to the point at which the property is relet or the point at which the property could have been relet as decided by the Court.
This is obviously open to market demand but if there was no break clause you would be looking at two to three months, possibly more depending on the rental market where you are.
The alternative if you anticipate that you are to leave earlier than a break clause or the fixed term is to find a replacement tenant yourself. The landlord is under a duty not to refuse a reasonable replacement tenant that you can produce to him to take over the responsibilities of the tenancy agreement. If he does unreasonably refuse you can simply terminate the tenancy and leave.
Also, if you are to leave and cannot find a replacement tenant immediately or do not have the time then you can take the landlord's thoughts on a surrender of tenancy. This would be in writing signed by you both, with each retaining a copy stating that the tenancy has been surrendered and you acknowledge the cessation of each other's mutual responsibilities under it. He would likely require some money for this.
You should attempt to negotiate an open ended break clause in the tenancy operable by you only after the time at which your work is not guaranteed. If they resist, say, one months notice offer them two and see how you get on.
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