Go to the police and fill out a report, it is stealing. The only way they may retain goods if they have a court order. I had an org tried to hold my tool box after I left them, police said it attempt to seal if they wont let me in the following day they charge them with that.
Go in tomorrow ask for it, if they wont give it back straight to cop shop. That way it will not cost you anything, second thing is go to your local member and have him send a letter to the aviation minster, then sit back and watch the action