How to explain away that Holiday Villa you bought using undeclared income

During a tax investigation an inspector from HMRC asks the taxpayer how he managed to afford such a luxurious holiday villa with such little income.  The taxpayer responded with this story:

"While out fishing last summer, I caught a large golden fish. When I took it off the hook, the fish opened its mouth and said that he was a magical fish. This magical fish promised me that if I threw him back into the sea he would reward me with the most luxurious villa I had ever seen. After thinking about it for a moment, I said "OK" and threw the magic fish back into the sea. When I walked back up from the sea I could see the villa, just as the fish had promised."

The Inspector laughed and asked how the taxpayer expected to prove such an unbelievable story?

The taxpayer said "Well, you can see the villa, can't you?"