Yup, the costs to the LL don't even bear thinking about.Section 21 most landlord's can deal with but the possibility of having to vett , signing in and signing out new tennants every 2 months could be the straw that just broke the Camels back - why have you introduced periodic tenancy agreements that means you can move in for 2 months and instantly give notice .
In fact it'd be completely pointless to do so.