While the present building, known as the Hall of Commerce when it was built in 1845, came after the time of Charles Jobson, he would have worked in the 14th century wooden building that was on the same site before.
The Borough Arms that adorn the top of the facade are thought to have been made at the time of King John giving Ipswich its first charter around 1200.
The cache is micro, magnetic and hidden within sight of the building and the docks. You don't need to enter the grounds in order to retrieve it.