Actually , something very similar to this happend in the States a few years back, except it was cash instead of a Van Gogh.
The original homeowner found out about it because the new home owner was bragging about finding money in the walls during a reno. Old homeowner confronted new homeowner and tried to work something out (i.e. 50/50)
New homeowner told old homeowner to suck an egg.
Old homeowner took new homeowner to court and....
Got 100%.
Same deal here; as the new owner, be nice. Karma can be cruel, and she will come for you.
Wow that is a pretty amazing story. Though it is kind of strange that the old home owner would get to keep 100%. Doesn't really make sense since it isn't their property. Though the new home owner is at fault for openly bragging about what they had found..