Will's Story about a Wheelbarrow

Here's a story set just after the second world war at Portsmouth dockyard.
One day Ministry of Defence policeman [Mod plod] stopped a worker who was walking out of the dockyard gates pushing a wheelbarrow with a suspicious looking package in it. The Mod plod opened the package and found it contained nothing but some old bits of rubbish, sawdust and floor-sweepings.
The next day he stopped the same worker who was again pushing a wheelbarrow containing a suspicious looking package. Once more it contained nothing of any value.
The same thing happened several days on the trot, until the policeman finally said, 'OK, I give up. I know you are up to something, but I just can't tell what. Please, I promise not to arrest you, but put me out of my misery; tell me what you are stealing.'
'Wheelbarrows, 'smiled the worker, 'I'm stealing wheelbarrows.'