It is probably best not to rely solely on the centre lock provided. If the door is the only means of entry to the garage, it should be fitted with a hasp and staple, together with a padlock as above. There are also mortice locks specially designed for garage doors that close into the side frame. Before fitting, take note of the gap between the door and the frame, as some of the bolts have only a short throw.
If there is another exit/entrance, and the door opens along runners, consider drilling through one of the runners just behind the wheel in the closed position, inserting a long bolt to stop the wheel moving along the runners. If the door operates on a hinged bar and not runners, drill through the side metal frame and pass a padlock through this hole and around the hinged bar to stop the door lifting.
For more advice and a professional service call Horsham Locksmiths now on 01403 261466