Okay you're only insured up to £46 through recorded delivery but I think you should ring the police if the royal mail are saying there is nothing they can do about it.
A theft has taken place, you can show you handed it to a royal mail employee (you've got a reciept), it was in the royal mail hands until it reached its destination (or didn't). You're prepared to make a statement that it was in the box when posted, person who received it is prepared to make a statement about the state of the box when it arrived, they know where its been because of the tracking. Police may be relucatant to get involved but it is their job!