Depends what kind of lease you mean. Do you want to let the flat, so you can regain possession at a later date? If so, all you need is an Assured Shorthold Tenancy Agreement. Many letting agents will draw one up for you as part of the letting service. It will cost you - but is well worth appointing an agent, as they will help you through all the steps you need to take to ensure the flat is in lettable condition and that you are fulfilling the legal requirements of being a Landlord.
One thing puzzles me, though - Freehold flats are very rare. Flats are usually held Long Leasehold (sometimes 999 years, which is effectively Freehold, but not the same thing). If your flat is Leasehold, you will need to check first that subletting is allowed.