Nothing to do with the agent you will need to speak to your solicitor/conveyancer.. The contract you exchanged and completed on was done on the basis of vacant possession and the property should be cleared of any items not on the fixtures and fittings list.. you can try and speak to your solictor and see if there is any come back but I've never known anyone successful in doing so

The solictor I recommend advises property/garages/sheds etc are inspected before and after exchange as once its completed its going to be a mission guess you live and learn