A Fixed Term lease is, as it says on the label - for a fixed term. A landlord cannot break the contract unless the tenant is in breach of their obligations or there is a suitable break clause.
Tenants, on the other hand, are allowed to assign a lease and thus break it legally.
It must also be remembered that once a tenant has been in occupancy of the property for six months, they are entitled to remain for a total of 4 years without signing a new lease.
A landlord may only use the grounds provided for in the RTA 2004 when the lease becomes a Part 4 tenancy. However, there is a difference of opinion as to whether a valid Notice of Termination may be served on the tenants during a fixed term lease.
That is to say, if a landlord serves a NoT six weeks prior to the expiry of the fixed term, giving notice of 35 days which expire at the same time as the fixed term (or even a days or two after the fixed term expires, is it legally valid as the Notice was served during the Fixed term?
Some say yes it is valid and other say it is not valid and a landlord must wait until the fixed term expires and enters into a Part 4 tenancy before the Notice is valid.
IMHO, it is valid to serve a NoT during a fixed term lease for several reasons:
1. after six months Part 4 rights become effective but precedence is given to a fixed term agreement as it is more secure than just a Part 4.
2. A NoT may be served during a Part 4 tenancy which will expire after the Part 4 tenancy has become a Further Part 4 - that is a notice served (with 112 days notice) six weeks before the Part 4 expires (i.e. six weeks before the the end of the 4th year. The RTA 2004 rules that the Notice does not expire at the end of the Part 4 tenancy but must continue into the Further Part 4 tenancy, until the 112 days have expired.