If you no longer live at the property you are complaining about, you must have had the landlord and tenant relationship at the time the complaint issue arose.
You should complain as soon as you become aware of the complaint issue.
The Ombudsman’s Complaint Handling Code says that complaints must be raised within a reasonable time, and this is usually within 12 months of the date of the issue arising. The Ombudsman may decide not to investigate a complaint where it was raised more than 12 months after the complaint issue arose.