I had never seen anything like this before and was unfamiliar with the custom, but in any case, my children thought it would be fun to add some coins of their own. There was a small rock left on one of the branches by another passer-by and we used it to hammer in a couple of coins.
An internet search later revealed that there are a number of these "Money Trees" mainly around Yorkshire and the Lake District. The coins are hammered into the tree in a similar way as they would be tossed into a well - to make a wish or for good luck. It is even believed by some that the practice can cure all kinds of illness!