If your house insurance covers theft from the garden, you'll need a police crime number for the insurance company.

If you don't report it, then the police have no idea it's happened, and therefore don't have a true idea of crime in your area. there won't be any push for resources in your area because, of course, there isn't any crime.

There is a remote chance that they'll find the perpetrator, especially if they're already looking at him for something else. For example, my father came home to find all his valuables in a pillow case on the landing. He rang the police, they said "we think we've got him" - they had a guy on bail doing his daily report (in between collecting my dad's valuables and going back to collect them), and they noticed my father's gold watch on his wrist, so they hung on to him.

So - remote chance of getting it back. But even if the chance is slim, it's still worth getting it on the record.