This is interesting. I've always been given, along with an initial rental agreement, a letter dated for the end of the tenancy that asks me to leave the property. It was my understanding that this is enough for the agent to easily eject you at the end of the agreement period unless you follow what they want.
Serving you notice before you've even moved in sounds like a pretty shady practise to me, and one that I'd be inclined to challenge before I signed anything.
If they attempt that sorts of trick before you've even paid them a penny, that doesn't bode well for the rest of the tenancy in my opinion!
