It was hidden in London. Here is the mail from Paul who found it:
This morning I got up very early and got the train to (1 hour and 30 mins away) and then a tube train to Regent's Park. I stumbled on the spot I saw in your photograph and then quickly went in the undergrowth not expecting to find anything! I was so excited when I found the tube with "The Treasure" written on it!
I am now back home and wanted to write to you as soon as possible to say thank you for the wonderful drawing and the excitement of a great adventure.
Thank you my friend.