2 Sainsbury's Shopping Trolley's left in my garden by students.

Have asked Serco to remove 2 shopping trolley's that have been left in my front garden by students. I am the landlord and live away from the area. I was contacted by a neighbour regarding this and I am annoyed that even though I have given them verbal permission they say they wont pick them up due to insurance purposes. This goes against government guidlines, perhaps the council should read the Gov Website. They are about six foot away from a publc pavement. They are the property of Sainsbury's so perhaps they could pick them up.