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My Tenants gave Notice - then didn't leave. 

How do I evict them?

You need to apply to the court for a possession order.  There is no notice required for such an order as the tenancy is now over and your former tenants are now trespassers.
The court forms you require are N5 & n120 and the fee is currently £175.  Unlike a tenant possession claim, a trespass claim will be held at quite short notice.
Once a possession order is granted, and presuming the former tenants do not surrender the tenancy, you will need to arrange bailiffs to remove them.
It is important that you do not accept rent off your former tenants, and thereby create a new tenancy.  If you accept any payment in lieu of rent you should mark the receipt at mesne profit.