You are going to have to give them Notice to vacate
At the end of the notice period you will nee do take them to court if they do not leave and also you can claim all unpaid rent up to the date they leave.
In your application you can either ask for them to remove the shed or reduce the amount owing based on the value of the shed. This is your only choice.
You will need to go to court because from what you are telling me I suspect they will not leave.
You can do this on your own but you will have to make sure you do everything in writing.
I hope this makes sense and is of assistance.
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