Deed of assignment or new tenancy?

Tenancy Types and Management

Two of the tenants are moving out this week end and they have found two suitable people to take their place.

What is the best way of dealing with this?

I usually accept a letter from the tenants which states that they wish to surrender their tenancy and I then respond by giving to them a letter agreeing to this and then create a new tenancy with the people who wish to remain and the newcomers.

However I wonder if this is ok as some of the current occupants will not actually move out of the property or would it be better to have a deed of assignment?
I would be very grateful for your advice