Shouty Grandad and the Garden Fork

Last week at sailing I met up with one of my neighbours, her daughter was also sailing. We got chatting and it transpired that a week or so before, some horrible 'sorts' had burgled her garden shed, various items were stolen.
It suddenly occurred to me, a couple of weeks ago, I had spotted a garden fork laying on top of the garages near our house. Fearing the local oiks would use it to stab holes in the tender garage roofs, I confiscated it. That Wednesday, when Shouty Grandad came round, I told him about the fork, he took a shine to it and carried it off home.
It seems the fork was stolen from my neighbour's shed. The question is, how do I get it back off Shouty Grandad?